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

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 <wqu@suse.com>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>