Update net/openafs to 1.6.15, a security release

OPENAFS-SA-2015-007 (CVE-2015-7762, CVE-2015-7763): rx ACK packets
leak plaintext of previous packets.

Approved by: feld (ports-secteam, ports committer)
Security: OPENAFS-SA-2015-007, CVE-2015-7762, CVE-2015-7763