ext4: fix null-ptr-deref in '__ext4_journal_ensure_credits'

ext4: fix null-ptr-deref in '__ext4_journal_ensure_credits'

We got issue as follows when run syzkaller test:
[ 1901.130043] EXT4-fs error (device vda): ext4_remount:5624: comm syz-executor.5: Abort forced by user
[ 1901.130901] Aborting journal on device vda-8.
[ 1901.131437] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.16: Detected aborted journal
[ 1901.131566] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.11: Detected aborted journal
[ 1901.132586] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.18: Detected aborted journal
[ 1901.132751] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.9: Detected aborted journal
[ 1901.136149] EXT4-fs error (device vda) in ext4_reserve_inode_write:6035: Journal has aborted
[ 1901.136837] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-fuzzer: Detected aborted journal
[ 1901.136915] ==================================================================
[ 1901.138175] BUG: KASAN: null-ptr-deref in __ext4_journal_ensure_credits+0x74/0x140 [ext4]
[ 1901.138343] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.13: Detected aborted journal
[ 1901.138398] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.1: Detected aborted journal
[ 1901.138808] Read of size 8 at addr 0000000000000000 by task syz-executor.17/968
[ 1901.138817]
[ 1901.138852] EXT4-fs error (device vda): ext4_journal_check_start:61: comm syz-executor.30: Detected aborted journal
[ 1901.144779] CPU: 1 PID: 968 Comm: syz-executor.17 Not tainted 4.19.90-vhulk2111.1.0.h893.eulerosv2r10.aarch64+ #1
[ 1901.146479] Hardware name: linux,dummy-virt (DT)
[ 1901.147317] Call trace:
[ 1901.147552]  dump_backtrace+0x0/0x2d8
[ 1901.147898]  show_stack+0x28/0x38
[ 1901.148215]  dump_stack+0xec/0x15c
[ 1901.148746]  kasan_report+0x108/0x338
[ 1901.149207]  __asan_load8+0x58/0xb0
[ 1901.149753]  __ext4_journal_ensure_credits+0x74/0x140 [ext4]
[ 1901.150579]  ext4_xattr_delete_inode+0xe4/0x700 [ext4]
[ 1901.151316]  ext4_evict_inode+0x524/0xba8 [ext4]
[ 1901.151985]  evict+0x1a4/0x378
[ 1901.152353]  iput+0x310/0x428
[ 1901.152733]  do_unlinkat+0x260/0x428
[ 1901.153056]  __arm64_sys_unlinkat+0x6c/0xc0
[ 1901.153455]  el0_svc_common+0xc8/0x320
[ 1901.153799]  el0_svc_handler+0xf8/0x160
[ 1901.154265]  el0_svc+0x10/0x218
[ 1901.154682] ==================================================================

This issue may happens like this:
	Process1                               Process2
ext4_evict_inode
  ext4_journal_start
   ext4_truncate
     ext4_ind_truncate
       ext4_free_branches
         ext4_ind_truncate_ensure_credits
	   ext4_journal_ensure_credits_fn
	     ext4_journal_restart
	       handle->h_transaction = NULL;
                                           mount -o remount,abort  /mnt
					   -> trigger JBD abort
               start_this_handle -> will return failed
  ext4_xattr_delete_inode
    ext4_journal_ensure_credits
      ext4_journal_ensure_credits_fn
        __ext4_journal_ensure_credits
	  jbd2_handle_buffer_credits
	    journal = handle->h_transaction->t_journal; ->null-ptr-deref

Now, indirect truncate process didn't handle error. To solve this issue
maybe simply add check handle is abort in '__ext4_journal_ensure_credits'
is enough, and i also think this is necessary.

Cc: stable@kernel.org
Signed-off-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/r/20211224100341.3299128-1-yebin10@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>