x86: move rdtsc_barrier() into the TSC vread method

x86: move rdtsc_barrier() into the TSC vread method

commit 7d96fd41cadc55f4e00231c8c71b8e25c779f122 upstream.

The *fence instructions were moved to vsyscall_64.c by commit
cb9e35dce94a1b9c59d46224e8a94377d673e204.  But this breaks the
vDSO, because vread methods are also called from there.

Besides, the synchronization might be unnecessary for other
time sources than TSC.

[ Impact: fix potential time warp in VDSO ]

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
LKML-Reference: <9d0ea9ea0f866bdc1f4d76831221ae117f11ea67.1243241859.git.ptesarik@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>