s3: smbd: Restart reading the incoming SMB2 fd when the send queue is drained.

s3: smbd: Restart reading the incoming SMB2 fd when the send queue is drained.

When the send queue grows greater than xconn->smb2.credits.max/16,
smbd_smb2_request_next_incoming() doesn't allocate a new request in state->req.

After smbd_smb2_io_handler() is called, it marks the fd not readable as
state->req == NULL, and never marks it readable again.

Fix by calling smbd_smb2_request_next_incoming() to restart
reads inside smbd_smb2_flush_send_queue() which drains the
send queue.

Reported by <chen.yehua@h3c.com>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12608

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri Mar  3 02:23:20 CET 2017 on sn-devel-144