btrfs: show correct warning if can't read data reloc tree

If a filesystem is missing its data reloc tree, we get something like
this in dmesg:

BTRFS warning (device loop11): failed to read root (objectid=4): -2
BTRFS error (device loop11): open_ctree failed: -2

objectid is BTRFS_DEV_TREE_OBJECTID, but this should actually be the
value of BTRFS_DATA_RELOC_TREE_OBJECTID.

btrfs_read_roots() prints location.objectid on failure, but this isn't
set when reading the data reloc tree. Set location.objectid to the
correct value on failure, so that the error message makes sense.

Reviewed-by: Qu Wenruo <[e-mail]>
Signed-off-by: Mark Harmstone <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/disk-io.c

···
 				 BTRFS_DATA_RELOC_TREE_OBJECTID, true);
 	if (IS_ERR(root)) {
 		if (!btrfs_test_opt(fs_info, IGNOREBADROOTS)) {
+			location.objectid = BTRFS_DATA_RELOC_TREE_OBJECTID;
 			ret = PTR_ERR(root);
 			goto out;
 		}