bcachefs: Set bch_inode_unpacked.bi_snapshot in old inode path

This fixes a fsck bug on a very old filesystem (pre mainline merge).

Fixes: 72350ee0 ("bcachefs: Kill snapshot arg to fsck_write_inode()")
Reported-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>