Btrfs: Fix a trivial warning using max() of u64 vs ULL.

A small warning popped up on ia64 because inode-map.c was comparing a
u64 object id with the ULL FIRST_FREE_OBJECTID.  My first thought was
that all the OBJECTID constants should contain the u64 cast because
btrfs code deals entirely in u64s.  But then I saw how large that was,
and figured I'd just fix the max() call.
Signed-off-by: NJoel Becker <joel.becker@oracle.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/inode-map.c

@@ -79,7 +79,7 @@ int btrfs_find_free_objectid(struct btrfs_trans_handle *trans,
 	}
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
-	search_start = max(search_start, BTRFS_FIRST_FREE_OBJECTID);
+	search_start = max(search_start, (u64)BTRFS_FIRST_FREE_OBJECTID);
 	search_key.objectid = search_start;
 	search_key.type = 0;
 	search_key.offset = 0;