smb: smbdirect: introduce smbdirect_map_sges_from_iter() and helper functions

smb: smbdirect: introduce smbdirect_map_sges_from_iter() and helper functions

These are basically copies of smb_extract_iter_to_rdma() and its helpers
in the client, which will be replaced in the next steps.

The goal is to use them also in the server, which will simplify a lot.

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: David Howells <dhowells@redhat.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>