vsock: introduce vsock_pending_to_accept() helper

vsock: introduce vsock_pending_to_accept() helper

Add vsock_pending_to_accept() to move a socket directly from the
pending list to the accept queue in a single operation, avoiding
the sock_put/sock_hold dance and the sk_acceptq_removed()/
sk_acceptq_added() pair that would otherwise be needed when
calling vsock_remove_pending() followed by vsock_enqueue_accept().

Use it in vmci_transport_recv_connecting_server() where a completed
handshake transitions the socket from pending to accept queue.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Raf Dickson <rafdog35@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260612045216.105796-2-rafdog35@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>