cifsd: add server handler for central processing and tranport layers

This adds server handler for central processing,
transport layers(tcp, rdma, ipc) and a document describing cifsd
architecture.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>