ext2: avoid drop_nlink() during unlink of zero-nlink inode in ext2_unlink()

ext2: avoid drop_nlink() during unlink of zero-nlink inode in ext2_unlink()

ext2_unlink() calls inode_dec_link_count() unconditionally, which
invokes drop_nlink(). If the inode was loaded from a corrupted disk
image with i_links_count == 0, drop_nlink()
triggers WARN_ON(inode->i_nlink == 0)

Follow the ext4 pattern from __ext4_unlink(): check i_nlink before
decrementing. If already zero, skip the decrement.

Signed-off-by: Ziyi Guo <n7l8m4@u.northwestern.edu>
Link: https://patch.msgid.link/20260211022052.973114-1-n7l8m4@u.northwestern.edu
Signed-off-by: Jan Kara <jack@suse.cz>