smb: server: move iov_iter_kvec() out of smb_direct_post_send_data()

smb: server: move iov_iter_kvec() out of smb_direct_post_send_data()

This will allow us to make the code more generic in order
to move it to common with the client.

Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>