powerpc/mm: factor out creating/removing linear mapping

We want to stop abusing memory hotplug infrastructure in memtrace code
to perform allocations and remove the linear mapping. Instead we will use
alloc_contig_pages() and remove the linear mapping manually.

Let's factor out creating/removing the linear mapping into
arch_create_linear_mapping() / arch_remove_linear_mapping() - so in the
future, we might be able to have whole arch_add_memory() /
arch_remove_memory() be implemented in common code.

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/20201111145322.15793-4-david@redhat.com