As cmsg.raid_slot is le32, comparing for >0 is not meaningful.

So introduce cpu-endian 'raid_slot' and only assign to cmsg.raid_slot
when we know value is valid.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.com>