powerpc/mm: print warning in arch_remove_linear_mapping()

Let's print a warning similar to in arch_add_linear_mapping() instead of
WARN_ON_ONCE() and eventually crashing the kernel.

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/20201111145322.15793-6-david@redhat.com