x86/xen: Simplify set_aliased_prot()

A year ago, via the following commit:

  aa1acff3 ("x86/xen: Probe target addresses in set_aliased_prot() before the hypercall")

I added an explicit probe to work around a hypercall issue.  The code can
be simplified by using probe_kernel_read().

No change in functionality.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Vrabel <dvrabel@cantab.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel <xen-devel@lists.xen.org>
Link: http://lkml.kernel.org/r/0706f1a2538e481194514197298cca6b5e3f2638.1464129798.git.luto@kernel.org


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