x86: reuse asm-generic/barrier.h

x86: reuse asm-generic/barrier.h

BugLink: http://bugs.launchpad.net/bugs/1616677
As on most architectures, on x86 read_barrier_depends and
smp_read_barrier_depends are empty.  Drop the local definitions and pull
the generic ones from asm-generic/barrier.h instead: they are identical.

This is in preparation to refactoring this code area.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
(cherry picked from commit 300b06d4555305dc227748674f75970f2f84c224)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>