f2fs: avoid null pointer access when handling IPU error

f2fs: avoid null pointer access when handling IPU error

 Unable to handle kernel NULL pointer dereference at virtual address 000000000000001a
 pc : f2fs_inplace_write_data+0x144/0x208
 lr : f2fs_inplace_write_data+0x134/0x208
 Call trace:
  f2fs_inplace_write_data+0x144/0x208
  f2fs_do_write_data_page+0x270/0x770
  f2fs_write_single_data_page+0x47c/0x830
  __f2fs_write_data_pages+0x444/0x98c
  f2fs_write_data_pages.llvm.16514453770497736882+0x2c/0x38
  do_writepages+0x58/0x118
  __writeback_single_inode+0x44/0x300
  writeback_sb_inodes+0x4b8/0x9c8
  wb_writeback+0x148/0x42c
  wb_do_writeback+0xc8/0x390
  wb_workfn+0xb0/0x2f4
  process_one_work+0x1fc/0x444
  worker_thread+0x268/0x4b4
  kthread+0x13c/0x158
  ret_from_fork+0x10/0x18

Fixes: 955772787667 ("f2fs: drop inplace IO if fs status is abnormal")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>