Btrfs-progs: skip opening all devices with restore

Btrfs-progs: skip opening all devices with restore

When we go to fixup the dev items after a restore we scan all existing devices.
If you happen to be a btrfs developer you could possibly open up some random
device that you didn't just restore onto, which gives you weird errors and makes
you super cranky and waste a day trying to figure out what is failing.  This
will make it so that we use the fd we've already opened for opening our ctree.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>