bcachefs: fix backpointer_to_text() when dev does not exist

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

fs/bcachefs/backpointers.c

@@ -68,9 +68,11 @@ void bch2_backpointer_to_text(struct printbuf *out, const struct bch_backpointer
 
 void bch2_backpointer_k_to_text(struct printbuf *out, struct bch_fs *c, struct bkey_s_c k)
 {
+	if (bch2_dev_exists2(c, k.k->p.inode)) {
 		prt_str(out, "bucket=");
 		bch2_bpos_to_text(out, bp_pos_to_bucket(c, k.k->p));
 		prt_str(out, " ");
+	}
 
 	bch2_backpointer_to_text(out, bkey_s_c_to_backpointer(k).v);
 }