ext4: remove dead check in mb_buddy_mark_free

We always adjust first to even number and adjust last to odd number, so
first == last will never happen. Remove this dead check.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-13-shikemeng@huaweicloud.com


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