asm-generic: introduce be48 unaligned accessors

asm-generic: introduce be48 unaligned accessors

The NVMe protocol extended the data integrity fields with unaligned
48-bit reference tags. Provide some helper accessors in
preparation for these.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220303201312.3255347-4-kbusch@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>