The NBD protocol defines a message for zeroing out a region of an export

Add support to the kernel driver for that message.
Signed-off-by: Wouter Verhelst <w@uter.be>
Cc: Eric Blake <eblake@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240812133032.115134-3-w@uter.beSigned-off-by: Jens Axboe <axboe@kernel.dk>