fs: btrfs: do not fail when offset of a ROOT_ITEM is not -1

When the btrfs_read_fs_root() function is searching a ROOT_ITEM with
location key offset other than -1, it currently fails via BUG_ON.

The offset can have other value than -1, though. This can happen for
example if a subvolume is renamed:

  $ btrfs subvolume create X && sync
  Create subvolume './X'
  $ btrfs inspect-internal dump-tree /dev/root | grep -B 2 'name: X$
        location key (270 ROOT_ITEM 18446744073709551615) type DIR
        transid 283 data_len 0 name_len 1
        name: X
  $ mv X Y && sync
  $ btrfs inspect-internal dump-tree /dev/root | grep -B 2 'name: Y$
        location key (270 ROOT_ITEM 0) type DIR
        transid 285 data_len 0 name_len 1
        name: Y

As can be seen the offset changed from -1ULL to 0.

Do not fail in this case.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: David Sterba <dsterba@suse.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Tom Rini <trini@konsulko.com>