Directly use fault_in_pages_readable instead of manual __get_user code. Fix
warning treated as error with W=1:

  arch/powerpc/kernel/kvm.c:675:6: error: variable ‘tmp’ set but not used [-Werror=unused-but-set-variable]
Suggested-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Reviewed-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>