lightnvm: pblk: Replace guid_copy() with export_guid()/import_guid()

There is a specific API to treat raw data as GUID, i.e. export_guid()
and import_guid(). Use them instead of guid_copy() with explicit casting.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Matias Bjørling <matias.bjorling@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>