powerpc/mm: Limit resize_hpt_for_hotplug() call to hash guests only

During memory hotplug and unplug, resize_hpt_for_hotplug() gets called
for both hash and radix guests but it should be called only for hash
guests. Though the call does nothing in the radix guest case, it is
cleaner to push this call into hash specific memory hotplug routines.

Reported-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200727095704.1432916-1-bharata@linux.ibm.com