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