nbd: move s->state under requests_lock

nbd: move s->state under requests_lock

Remove the confusing, and most likely wrong, atomics.  The only function
that used to be somewhat in a hot path was nbd_client_connected(),
but it is not anymore after the previous patches.

The same logic is used both to check if a request had to be reissued
and also in nbd_reconnecting_attempt().  The former cases are outside
requests_lock, while nbd_reconnecting_attempt() does have the lock,
therefore the two have been separated in the previous commit.
nbd_client_will_reconnect() can simply take s->requests_lock, while
nbd_reconnecting_attempt() can inline the access now that no
complicated atomics are involved.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-8-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>