smb: smbdirect: introduce smbdirect_socket.c to be filled

smb: smbdirect: introduce smbdirect_socket.c to be filled

Over time smbdirect_socket.c will get more and more functions
which will be included in fs/smb/client/smbdirect.c and
fs/smb/server/transport_rdma.c via
fs/smb/common/smbdirect/smbdirect_all_c_files.c
in order to allow tiny steps in the direction of moving to
a few exported functions from an smbdirect.ko.
That's why __maybe_unused is added for now it will
be removed at the end of the road to common code.

Note the Copyright (C) 2017, Microsoft Corporation is added
as a lot of functions from fs/smb/client/smbdirect.c will
be moved into this file soon and I don't want to forget
about adding it.

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>