cifs: move handling of signed connections into separate function

Move the sanity checks for signed connections into a separate function.
SMB2's was a cut-and-paste job from CIFS code, so we can make them use
the same function.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>