bcachefs: Fix bch2_delete_dead_inodes()

 - the fsck_err() check for the filesystem being clean was incorrect,
   causing us to always fail to delete unlinked inodes
 - if a snapshot had been taken, the unlinked inode needs to be
   propagated to snapshot leaves so the unlink can happen there - fixed.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>