cifs: make error on lack of a unc= option more explicit

Error out with a clear error message if there is no unc= option. The
existing code doesn't handle this in a clear fashion, and the check for
a UNCip option with no UNC string is just plain wrong.

Later, we'll fix the code to not require a unc= option, but for now we
need this to at least clarify why people are getting errors about DFS
parsing. With this change we can also get rid of some later NULL pointer
checks since we know the UNC and UNCip will never be NULL there.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>