[CIFS] implement upcalls for SPNEGO blob via keyctl API

Add routines to handle upcalls to userspace via keyctl for the purpose
of getting a SPNEGO blob for a particular uid and server combination.

Clean up the Makefile a bit and set it up to only compile cifs_spnego
if CONFIG_CIFS_UPCALL is set. Also change CONFIG_CIFS_UPCALL to depend
on CONFIG_KEYS rather than CONFIG_CONNECTOR.

cifs_spnego.h defines the communications between kernel and userspace
and is intended to be shared with userspace programs.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>