[CIFS] allow cifs_calc_signature2 to deal with a zero length iovec

Currently, cifs_calc_signature2 errors out if it gets a zero-length
iovec. Fix it to silently continue in that case.

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