x86: Fix vsyscall on gcc 4.5 with -Os

This fixes the -Os breaks with gcc 4.5 bug.  rdtsc_barrier needs to be
force inlined, otherwise user space will jump into kernel space and
kill init.

This also addresses http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44129
I believe.
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
LKML-Reference: <20100618210859.GA10913@basil.fritz.box>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@kernel.org>

arch/x86/include/asm/system.h

@@ -451,7 +451,7 @@ void stop_this_cpu(void *dummy);
  *
  * (Could use an alternative three way for this if there was one.)
  */
-static inline void rdtsc_barrier(void)
+static __always_inline void rdtsc_barrier(void)
 {
 	alternative(ASM_NOP3, "mfence", X86_FEATURE_MFENCE_RDTSC);
 	alternative(ASM_NOP3, "lfence", X86_FEATURE_LFENCE_RDTSC);