powerpc: Fix boot on BOOK3S_32 with CONFIG_STRICT_KERNEL_RWX

On powerpc32, patch_instruction() is called by apply_feature_fixups()
which is called from early_init()

There is the following note in front of early_init():
 * Note that the kernel may be running at an address which is different
 * from the address that it was linked at, so we must use RELOC/PTRRELOC
 * to access static data (including strings).  -- paulus

Therefore, slab_is_available() cannot be called yet, and
text_poke_area must be addressed with PTRRELOC()

Fixes: 95902e6c8864 ("powerpc/mm: Implement STRICT_KERNEL_RWX on PPC32")
Cc: stable@vger.kernel.org # v4.14+
Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/code-patching.c

@@ -21,6 +21,7 @@
 #include <asm/tlbflush.h>
 #include <asm/page.h>
 #include <asm/code-patching.h>
+#include <asm/setup.h>
 
 static int __patch_instruction(unsigned int *addr, unsigned int instr)
 {
@@ -146,11 +147,8 @@ int patch_instruction(unsigned int *addr, unsigned int instr)
 	 * During early early boot patch_instruction is called
 	 * when text_poke_area is not ready, but we still need
 	 * to allow patching. We just do the plain old patching
-	 * We use slab_is_available and per cpu read * via this_cpu_read
-	 * of text_poke_area. Per-CPU areas might not be up early
-	 * this can create problems with just using this_cpu_read()
 	 */
-	if (!slab_is_available() || !this_cpu_read(text_poke_area))
+	if (!this_cpu_read(*PTRRELOC(&text_poke_area)))
 		return __patch_instruction(addr, instr);
 
 	local_irq_save(flags);