ext4: add missing unlock in ext4_check_descriptors() on error path

(cherry picked from commit 7ee1ec4ca30c6df8e989615cdaacb75f2af4fa6b)

If there group descriptors are corrupted we need unlock the block
group lock before returning from the function; else we will oops when
freeing a spinlock which is still being held.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>