cifs: Move string length definitions to uapi

The max string length definitions for user name, domain name, password,
and share name have been moved into their own header file in uapi so the
mount helper can use autoconf to define them instead of keeping the
kernel side and userland side definitions in sync manually.  The names
have also been standardized with a "CIFS" prefix and "LEN" suffix.

Signed-off-by: Scott Lovenberg <scott.lovenberg@gmail.com>
Reviewed-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Steve French <smfrench@gmail.com>