x86/mm/pat: Export pat_enabled()

Two Linux device drivers cannot work with PAT and the work
required to make them work is significant. There is not enough
motivation to convert these drivers over to use PAT properly,
the compromise reached is to let drivers that cannot be ported
to PAT check if PAT was enabled and if so fail on probe with a
recommendation to boot with the "nopat" kernel parameter.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Walls <awalls@md.metrocast.net>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1430425520-22275-4-git-send-email-mcgrof@do-not-panic.com
Link: http://lkml.kernel.org/r/1432628901-18044-14-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>