powerpc/powernv: Increase memory block size to 1GB on radix

Memory hot unplug on PowerNV radix hosts is broken. Our memory block
size is 256MB but since we map the linear region with very large
pages, each pte we tear down maps 1GB.

A hot unplug of one 256MB memory block results in 768MB of memory
getting unintentionally unmapped. At this point we are likely to oops.

Fix this by increasing our memory block size to 1GB on PowerNV radix
hosts.

Fixes: 4b5d62ca

 ("powerpc/mm: add radix__remove_section_mapping()")
Cc: stable@vger.kernel.org # v4.11+
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>