fstests: btrfs: a new test case to verify scrub and rescue=idatacsums

fstests: btrfs: a new test case to verify scrub and rescue=idatacsums

There is a kernel bug report that scrub will trigger a NULL pointer
dereference when rescue=idatacsums mount option is provided.

Add a test case for such situation, to verify kernel can gracefully
reject scrub when  there is no csum tree.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>