ext4: fix memory leak in ext4_mb_init_backend on error path.

Fix a memory leak discovered by syzbot when a file system is corrupted
with an illegally large s_log_groups_per_flex.

Reported-by:  <syzbot+aa12d6106ea4ca1b6aae@syzkaller.appspotmail.com>
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20210412073837.1686-1-phil@philpotter.co.uk


Signed-off-by: Theodore Ts'o <tytso@mit.edu>