nbd/client: Relax handling of large NBD_CMD_BLOCK_STATUS reply

nbd/client: Relax handling of large NBD_CMD_BLOCK_STATUS reply

The NBD spec is proposing a relaxation of NBD_CMD_BLOCK_STATUS
where a server may have the final extent per context give a
length beyond the original request, if it can easily prove that
subsequent bytes have the same status, on the grounds that a
client can take advantage of this information for fewer block
status requests.  Since qemu 2.12 as a client always sends
NBD_CMD_FLAG_REQ_ONE, and rejects a server that sends extra
length, the upstream NBD spec will probably limit this behavior
to clients that don't request REQ_ONE semantics; but it doesn't
hurt to relax qemu to always be permissive of this server
behavior, even if it continues to use REQ_ONE.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180503222626.1303410-1-eblake@redhat.com>
Reviewed-by:  Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>