bcachefs: bch_snapshot::btime

Add a field to bch_snapshot for creation time; this will be important
when we start exposing the snapshot tree to userspace.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -1149,6 +1149,7 @@ struct bch_snapshot {
 	__le32			tree;
 	__le32			depth;
 	__le32			skip[3];
+	bch_le128		btime;
 };
 
 LE32_BITMASK(BCH_SNAPSHOT_DELETED,	struct bch_snapshot, flags,  0,  1)

fs/bcachefs/snapshot.c

@@ -1053,6 +1053,8 @@ static int create_snapids(struct btree_trans *trans, u32 parent, u32 tree,
 		n->v.subvol	= cpu_to_le32(snapshot_subvols[i]);
 		n->v.tree	= cpu_to_le32(tree);
 		n->v.depth	= cpu_to_le32(depth);
+		n->v.btime.lo	= cpu_to_le64(bch2_current_time(c));
+		n->v.btime.hi	= 0;
 
 		for (j = 0; j < ARRAY_SIZE(n->v.skip); j++)
 			n->v.skip[j] = cpu_to_le32(bch2_snapshot_skiplist_get(c, parent));