cifs: smbd: take an array of reqeusts when sending upper layer data

To support compounding, __smb_send_rqst() now sends an array of requests to
the transport layer.
Change smbd_send() to take an array of requests, and send them in as few
packets as possible.

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>