bcachefs: Fix missing bounds checks in bch2_alloc_read()

We were checking that the alloc key was for a valid device, but not a
valid bucket.

This is the upgrade path from versions prior to bcachefs being mainlined.

Reported-by: syzbot+a1b59c8e1a3f022fd301@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -639,6 +639,16 @@ int bch2_alloc_read(struct bch_fs *c)
 				continue;
 			}
 
+			if (k.k->p.offset < ca->mi.first_bucket) {
+				bch2_btree_iter_set_pos(&iter, POS(k.k->p.inode, ca->mi.first_bucket));
+				continue;
+			}
+
+			if (k.k->p.offset >= ca->mi.nbuckets) {
+				bch2_btree_iter_set_pos(&iter, POS(k.k->p.inode + 1, 0));
+				continue;
+			}
+
 			struct bch_alloc_v4 a;
 			*bucket_gen(ca, k.k->p.offset) = bch2_alloc_to_v4(k, &a)->gen;
 			0;