ext4: fix possible leak of sbi->s_group_desc_leak in error path

Fixes: bfe0a5f4 ("ext4: add more mount time checks of the superblock")
Reported-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 4.18