ext4: reorganize if statement of ext4_mb_release_context()

Reorganize the if statement of ext4_mb_release_context(), make it
easier to read.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Link: https://lore.kernel.org/r/5439ac6f-db79-ad68-76c1-a4dda9aa0cc3@gmail.com
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>