smb: client: let smbd_post_send_iter() call ib_dma_map_single() for the header first

This will simplify further changes, the important part is that
request->num_sge >= 1 is only set if request->sge[0].* is valid.

Note that ib_dma_sync_single_for_device() is called in
smbd_post_send() for each sge, so the device will still
see the packet header even if it's modified after calling
ib_dma_map_single().

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