btrfs: test snapshot create with invalid parent qgroup

Test that an invalid parent qgroup does not cause snapshot create to
force the FS readonly.

In btrfs, create_pending_snapshot() will go readonly on _any_ error return
from
btrfs_qgroup_inherit(). If qgroups are enabled, a user can crash their fs by
just making a snapshot and asking it to inherit from an invalid qgroup.

This patch does exactly that test. If the FS goes readonly that will be
reported and we will know that a regression was introduced.

The btrfs fix this patch relates to can be found at the following url:
http://thread.gmane.org/gmane.comp.file-systems.btrfs/54755

Thanks,
	--Mark

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/btrfs/121.out

@@ -0,0 +1,2 @@
+QA output created by 121
+Silence is golden