powerpc/sparse: Constify the address pointer in __get_user_nocheck()

In __get_user_nocheck, we create an intermediate pointer for the
user address we're about to fetch. We currently don't tag this
pointer as const. Make it const, as we are simply dereferencing
it, and it's scope is limited to the __get_user_nocheck macro.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>