xfstests: stat the dev we're removing to make sure its' really gone

xfstests: stat the dev we're removing to make sure its' really gone

I've been periodically failing btrfs/003 because my box sometimes takes a little
longer to unregister the device when we remove it and so the output from btrfs
dev show doesn't match what we are wanting since it still sees the device.  To
fix this just stat and sleep if we still see the device node and only continue
once udev or whatever actually removes the device node so that we don't get
random failures.

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>