Btrfs-progs: fix unresolved ref root message

Btrfs-progs: fix unresolved ref root message

btrfsck misinformed "unresolved ref root" message when there were several
snapshots in the file system. The patch(commit cfdd42686c70) tried to fix
this bug, but didn't fix it completely. If the metadata was stored in a
shared leaf of the tree, the problem would happen again. This patch fixes
it by another way, we don't check the relationship of the trees when we
traverse the fs/file tree, we just do the check when we merge the snapshot
records to the root cache.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>