[JENKINS:SECURITY-975] CSRF vulnerability and missing permission checks in Publish Over CIFS Plugin

Publish Over CIFS Plugin did not perform permission checks on a method implementing form validation.
This allowed users with Overall/Read access to Jenkins to initiate CIFS connections to an attacker specified host.

Additionally, this form validation method did not require POST requests, resulting in a CSRF vulnerability.

This form validation method now requires POST requests and Overall/Administer permissions.