MFC: r285113
If a "principal" argument isn't provided for a Kerberized NFS mount,
the kernel would generate a bogus one with a ":/<path>" suffix.
This would only occur for the case where there was no explicit
"principal" argument and the getaddrinfo() call in mount_nfs.c failed to a
return a cannonical name for the server.
This patch fixes this unusual case.