powerpc: regain entire stack space

thread_info is not anymore in the stack, so the entire stack
can now be used.

There is also no risk anymore of corrupting task_cpu(p) with a
stack overflow so the patch removes the test.

When doing this, an explicit test for NULL stack pointer is
needed in validate_sp() as it is not anymore implicitely covered
by the sizeof(thread_info) gap.

In the meantime, with the previous patch all pointers to the stacks
are not anymore pointers to thread_info so this patch changes them
to void*

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/asm-offsets.c

@@ -92,7 +92,6 @@ int main(void)
 	DEFINE(SIGSEGV, SIGSEGV);
 	DEFINE(NMI_MASK, NMI_MASK);
 #else
-	DEFINE(THREAD_INFO_GAP, _ALIGN_UP(sizeof(struct thread_info), 16));
 	OFFSET(KSP_LIMIT, thread_struct, ksp_limit);
 #ifdef CONFIG_PPC_RTAS
 	OFFSET(RTAS_SP, thread_struct, rtas_sp);