Do not use a non-literal format string

Now that UKERNEL's panic() is a proper varargs function (gerrit 9877),
we can use a literal format string "%s" to print the panic message.
clang warngs about a non-literal format string, and in some build
environments the warning becomes fatal via -Werror.

Change-Id: I7d8c9390e3edad7efa6ae1217f938b635423d23a
Reviewed-on: http://gerrit.openafs.org/10114
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>