bcachefs: Fix assert in bch2_backpointer_invalid()

Backpointers that point to invalid devices are caught by fsck, not
.key_invalid; so .key_invalid needs to check for them instead of hitting
asserts.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -44,6 +44,11 @@ int bch2_backpointer_invalid(struct bch_fs *c, struct bkey_s_c k,
 			     struct printbuf *err)
 {
 	struct bkey_s_c_backpointer bp = bkey_s_c_to_backpointer(k);
+
+	/* these will be caught by fsck */
+	if (!bch2_dev_exists2(c, bp.k->p.inode))
+		return 0;
+
 	struct bpos bucket = bp_pos_to_bucket(c, bp.k->p);
 	int ret = 0;