x86/traps: Use format string with panic() call

Building with -Wformat-nonliteral gives:

  arch/x86/kernel/traps.c:334:2: warning: format not a string literal and no format arguments [-Wformat-nonliteral]
    panic(message);

handle_stack_overflow() can only be called from two places (kernel/traps.c
and via inline asm in mm/fault.c), in both cases with a string not
containing format specifiers, so we might as well silence this warning
using "%s" as a format string.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181026222004.14193-1-linux@rasmusvillemoes.dk
Signed-off-by: Ingo Molnar <mingo@kernel.org>