f2fs: fix to avoid discard command leak

f2fs: fix to avoid discard command leak

 =============================================================================
 BUG discard_cmd (Tainted: G    B      OE  ): Objects remaining in discard_cmd on __kmem_cache_shutdown()
 -----------------------------------------------------------------------------

 INFO: Slab 0xffffe1ac481d22c0 objects=36 used=2 fp=0xffff936b4748bf50 flags=0x2ffff0000000100
 Call Trace:
  dump_stack+0x63/0x87
  slab_err+0xa1/0xb0
  __kmem_cache_shutdown+0x183/0x390
  shutdown_cache+0x14/0x110
  kmem_cache_destroy+0x195/0x1c0
  f2fs_destroy_segment_manager_caches+0x21/0x40 [f2fs]
  exit_f2fs_fs+0x35/0x641 [f2fs]
  SyS_delete_module+0x155/0x230
  ? vtime_user_exit+0x29/0x70
  do_syscall_64+0x6e/0x160
  entry_SYSCALL64_slow_path+0x25/0x25

 INFO: Object 0xffff936b4748b000 @offset=0
 INFO: Object 0xffff936b4748b070 @offset=112
 kmem_cache_destroy discard_cmd: Slab cache still has objects
 Call Trace:
  dump_stack+0x63/0x87
  kmem_cache_destroy+0x1b4/0x1c0
  f2fs_destroy_segment_manager_caches+0x21/0x40 [f2fs]
  exit_f2fs_fs+0x35/0x641 [f2fs]
  SyS_delete_module+0x155/0x230
  do_syscall_64+0x6e/0x160
  entry_SYSCALL64_slow_path+0x25/0x25

Recovery can cache discard commands, so in error path of fill_super(),
we need give a chance to handle them, otherwise it will lead to leak
of discard_cmd slab cache.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>