Suppress nonliteral format string warning/error

Clang doesn't like a nonliteral format string, and some kernel
builds (e.g., freebsd) are done with -Werror.  Use the standard
workaround for FreeBSD and UKERNEL builds by calling vsnprintf()
into a fixed buffer.

Remove the !defined(AFS_LINUX26_ENV) check, as it duplicates a
conditional around the entirety of osi_Panic().

Change-Id: If6287dd19604b78150c81febba8a59b73f56783c
Reviewed-on: http://gerrit.openafs.org/9880
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>