uapi nbd: add cookie alias to handle

The uapi <linux/nbd.h> header declares a 'char handle[8]' per request;
which is overloaded in English (are you referring to "handle" the
verb, such as handling a signal or writing a callback handler, or
"handle" the noun, the value used in a lookup table to correlate a
response back to the request).  Many user-space NBD implementations
(both servers and clients) have instead used 'uint64_t cookie' or
similar, as it is easier to directly assign an integer than to futz
around with memcpy.  In fact, upstream documentation is now
encouraging this shift in terminology:
https://github.com/NetworkBlockDevice/nbd/commit/ca4392eb2b

Accomplish this by use of an anonymous union to provide the alias for
anyone getting the definition from the uapi; this does not break
existing clients, while exposing the nicer name for those who prefer
it.  Note that block/nbd.c still uses the term handle (in fact, it
actually combines a 32-bit cookie and a 32-bit tag into the 64-bit
handle), but that internal usage is not changed by the public uapi,
since no compliant NBD server has any reason to inspect or alter the
64 bits sent over the socket.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20230410180611.1051618-3-eblake@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>