smb/client: fix rdma usage in smb2_async_writev()

rqst.rq_iter needs to be truncated otherwise we'll
also send the bytes into the stream socket...

This is the logic behind rqst.rq_npages = 0, which was removed in
"cifs: Change the I/O paths to use an iterator rather than a page list"
(d08089f6).

Cc: stable@vger.kernel.org
Fixes: d08089f6 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>