KVM: PPC: Book3S HV: Fix dirty map for hugepages

The dirty map that we construct for the KVM_GET_DIRTY_LOG ioctl has
one bit per system page (4K/64K).  Currently, we only set one bit in
the map for each HPT entry with the Change bit set, even if the HPT is
for a large page (e.g., 16MB).  Userspace then considers only the
first system page dirty, though in fact the guest may have modified
anywhere in the large page.

To fix this, we make kvm_test_clear_dirty() return the actual number
of pages that are dirty (and rename it to kvm_test_clear_dirty_npages()
to emphasize that that's what it returns).  In kvmppc_hv_get_dirty_log()
we then set that many bits in the dirty map.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>