nbd/server: push pending frames after sending reply

nbd/server: push pending frames after sending reply

qemu-nbd doesn't set TCP_NODELAY on the tcp socket.

Kernel waits for more data and avoids transmission of small packets.
Without TLS this is barely noticeable, but with TLS this really shows.

Booting a VM via qemu-nbd on localhost (with tls) takes more than
2 minutes on my system.  tcpdump shows frequent wait periods, where no
packets get sent for a 40ms period.

Add explicit (un)corking when processing (and responding to) requests.
"TCP_CORK, &zero" after earlier "CORK, &one" will flush pending data.

VM Boot time:
main:    no tls:  23s, with tls: 2m45s
patched: no tls:  14s, with tls: 15s

VM Boot time, qemu-nbd via network (same lan):
main:    no tls:  18s, with tls: 1m50s
patched: no tls:  17s, with tls: 18s

Future optimization: if we could detect if there is another pending
request we could defer the uncork operation because more data would be
appended.

Signed-off-by: Florian Westphal <fw@strlen.de>
Message-Id: <20230324104720.2498-1-fw@strlen.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>