bcachefs: Fix error path of bch2_link_trans()

In bch2_link_trans(), if bch2_inode_nlink_inc() fails, it needs to
call bch2_trans_iter_exit() in the error path.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>