powerpc/highmem: Change BUG_ON() to WARN_ON()

In arch/powerpc/mm/highmem.c, BUG_ON() is called only when
CONFIG_DEBUG_HIGHMEM is selected, this means the BUG_ON() is not vital
and can be replaced by a a WARN_ON().

At the same time, use IS_ENABLED() instead of #ifdef to clean a bit.

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