smb2_server: use sendmsg/recvmsg instead of writev/readv

smb2_server: use sendmsg/recvmsg instead of writev/readv

This avoids a few function calls inside the kernel
in order to reach sock_sendmsg() quicker:

    entry_SYSCALL_64_after_hwframe
       do_syscall_64
          __x64_sys_writev
             do_writev
                vfs_writev
                   do_iter_write
                      do_iter_readv_writev
                         sock_write_iter
                            sock_sendmsg

    entry_SYSCALL_64_after_hwframe
       do_syscall_64
          __x64_sys_sendmsg
             __sys_sendmsg
                ___sys_sendmsg
                   sock_sendmsg

As a side effect it will be useful for SMB-Direct invalidation
messages via msg->msg_control and CMSG_*.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>