xfstests: fix set-default test in btrfs/001

xfstests: fix set-default test in btrfs/001

We were actually testing this improperly, there was a bug in the set default
code so we weren't actually honoring the 0 subvolid properly.  To fix this we
need to get the subvolid for the subvol we want to set as the default and use
that in the command.  With this patch we now pass again with the fix for the 0
subvolid.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Tested-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>