smb: smbdirect: introduce smbdirect_rw.c with server rw code

smb: smbdirect: introduce smbdirect_rw.c with server rw code

This is basically contains the following functions copied from
the server: wait_for_rw_credits, calc_rw_credits, get_sg_list,
smb_direct_free_rdma_rw_msg, read_write_done, read_done,
write_done, smb_direct_rdma_xmit.

They got new names, some indentation/formatting changes,
some variable names are changed too.

They also only use struct smbdirect_socket instead of
struct smb_direct_transport.

But the logic is still the same. They will be used
by the server soon.

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>