[POWERPC] Make pSeries use the H_BULK_REMOVE hypervisor call

H_BULK_REMOVE lets us remove 4 entries from the MMU hash table with one
hypervisor call.  This uses it in pSeries_lpar_hpte_invalidate so we
can tear down mappings with fewer hypervisor calls.

Signed-off-by: Paul Mackerras <paulus@samba.org>