Btrfs-progs: fix incorrect root backref errors in fsck

Btrfs-progs: fix incorrect root backref errors in fsck

A user reported that fsck was complaining about unresolved refs for some
snapshots.  You can reproduce this by doing

mkfs.btrfs /dev/sdb
mount /dev/sdb /mnt
btrfs subvol snap /mnt/ /mnt/a
btrfs subvol snap /mnt/ /mnt/b
btrfs subvol del /mnt/a
umount /mnt
btrfsck /dev/sdb

and you'd get this

unresolved ref root 258 dir 256 index 2 namelen 1 name a error 600

because snapshot b has a dir item that points to a.  Except we encode in our
root ref the dirid of the ref holder, and if it doesn't match we just give it
back a empty directory since we can't hardlink directories.  This makes the
check in btrfsck bogus, when we delete a we remove the ref key for it so any
lookups into /mnt/b/a will just give a blank directory as it's supposed to.  Fix
this by only saying the backref is reachable if there is both a DIR_ITEM and a
REF_KEY for the given root.  With this patch I no longer see errors when running
this reproducer.  Thanks,

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