bcachefs: When fsck finds redundant snapshot keys, trigger snapshots cleanup

Fsck now checks for keys in different snapshot IDs that are now
redundant due to other snapshots being deleted - it needs to for its own
algorithms to not get confused.

When it detects this it should re-run the post snapshot deletion cleanup
- this patch does that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/errcode.h

@@ -7,6 +7,7 @@ enum {
 	OPEN_BUCKETS_EMPTY =	2048,
 	FREELIST_EMPTY,		/* Allocator thread not keeping up */
 	INSUFFICIENT_DEVICES,
+	NEED_SNAPSHOT_CLEANUP,
 };
 
 #endif /* _BCACHFES_ERRCODE_H */

fs/bcachefs/fsck.c

@@ -512,7 +512,7 @@ static int snapshots_seen_update(struct bch_fs *c, struct snapshots_seen *s,
 					bch2_btree_ids[btree_id],
 					pos.inode, pos.offset,
 					i->id, n.id, n.equiv);
-				return -EINVAL;
+				return -NEED_SNAPSHOT_CLEANUP;
 			}
 
 			return 0;
@@ -2388,7 +2388,9 @@ static int fix_reflink_p(struct bch_fs *c)
  */
 int bch2_fsck_full(struct bch_fs *c)
 {
-	return  bch2_fs_check_snapshots(c) ?:
+	int ret;
+again:
+	ret =   bch2_fs_check_snapshots(c) ?:
 		bch2_fs_check_subvols(c) ?:
 		bch2_delete_dead_snapshots(c) ?:
 		check_inodes(c, true) ?:
@@ -2399,6 +2401,13 @@ int bch2_fsck_full(struct bch_fs *c)
 		check_directory_structure(c) ?:
 		check_nlinks(c) ?:
 		fix_reflink_p(c);
+
+	if (ret == -NEED_SNAPSHOT_CLEANUP) {
+		set_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags);
+		goto again;
+	}
+
+	return ret;
 }
 
 int bch2_fsck_walk_inodes_only(struct bch_fs *c)