cifs: Fix invalid check in __cifs_calc_signature()

The following check would never evaluate to true:
  > if (i == 0 && iov[0].iov_len <= 4)

Because 'i' always starts at 1.

This patch fixes it and also move the header checks outside the for loop
- which makes more sense.

Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>