Hoist the libnvdimm helper as an inline helper to linux/uuid.h
using an auxiliary const variable uuid_null in lib/uuid.c.

[hch: also add the guid variant.  Both do the same but I'd like
to keep casts to a minimum]

The common helper uses the new abstract type uuid_t * instead of
u8 *.
Suggested-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
[hch: added guid_is_null]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>