smb: client: make use of smbdirect_map_sges_from_iter()

This is basically a copy of smb_extract_iter_to_rdma() moved
to common code.

Before we had the inconsistency we called ib_dma_unmap_single(),
while we mapped using ib_dma_map_page() in smb_set_sge().

Now ib_dma_unmap_page() is used for consistency.

It doesn't really matter as ib_dma_unmap_single() and
ib_dma_unmap_page() both operate
on dma_addr_t and dma_unmap_single_attrs() is just an
alias for dma_unmap_page_attrs().

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: 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>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>