An over-long path argument to gssd_syscall could overrun the stack sockaddr_un
buffer. Fix gssd_syscall to not permit that.
If an over-long path is provided, gssd_syscall now returns EINVAL.
It looks like PRIV_NFS_DAEMON isn't granted anywhere, so my best guess is that
this is likely only triggerable by root.
Reported by: Coverity
CID: 1006751
Sponsored by: EMC / Isilon Storage Division