cifs.upcall: try to use default credcache if we didn't find one

cifs.upcall: try to use default credcache if we didn't find one

Fedora is in the process of moving to KEYRING: credcaches which are not
currently handled by cifs.upcall. We could try to detect when they're in
use, but it's simpler and more robust to just try to use the default
credcache whenever we don't find a FILE: or DIR: cache.

Signed-off-by: Jeff Layton <jlayton@samba.org>