Btrfs-progs: fix reference check for roots in btrfsck

Btrfs-progs: fix reference check for roots in btrfsck

I noticed that I was getting these errors on a bigger file system with more
snapshots that had been removed.  This check is bogus since we won't inc
rec->found_ref if we don't find a REF_KEY _and_ a DIR_ITEM, so we only have to
worry about there being no references to a root if it actually has a root item.
If it doesn't then it's just referenced by things that will go no where anyway.
With this patch fsck no longer incorrectly complains about this file system
image I have.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>