nbd: code motion and function renaming

nbd: code motion and function renaming

Prepare for the next patch, so that the diff is less confusing.

nbd_client_connecting is moved closer to the definition point.

nbd_client_connecting_wait() is kept only for the reconnection
logic; when it is used to check if a request has to be reissued,
use the renamed function nbd_client_will_reconnect().  In the
next patch, the two cases will have different locking requirements.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-7-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>