powerpc/pseries: Flush lazy kernel mappings after unplug operations

This ensures that the translations for unmapped IO mappings or
unmapped memory are properly removed from the MMU hash table
before such an unplug. Without this, the hypervisor refuses the
unplug operations due to those resources still being mapped by
the partition.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>