powerpc/mm: remove linear mapping if __add_pages() fails in arch_add_memory()

Let's revert what we did in case something goes wrong and we return an
error - as already done on arm64 and s390x.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111145322.15793-8-david@redhat.com