powerpc/process: Replace an #ifdef CONFIG_PPC_BOOK3S_64 by IS_ENABLED()

This #ifdef CONFIG_PPC_BOOK3S_64 calls preload_new_slb_context()
when radix is not enabled.

radix_enabled() is always defined, and the prototype for
preload_new_slb_context() is always present, so the #ifdef
is unneeded.

Replace it by IS_ENABLED().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d31506ca9bac9def68cf7424eded63fdc4fb6660.1597643167.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/process.c

@@ -1780,10 +1780,8 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp)
 #ifdef CONFIG_PPC64
 	unsigned long load_addr = regs->gpr[2];	/* saved by ELF_PLAT_INIT */
 
-#ifdef CONFIG_PPC_BOOK3S_64
-	if (!radix_enabled())
+	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && !radix_enabled())
 		preload_new_slb_context(start, sp);
-#endif
 #endif
 
 	/*