KVM: pass kvm_memory_slot to gfn_to_page_many_atomic

The memory slot is already available from gfn_to_memslot_dirty_bitmap.
Isn't it a shame to look it up again?  Plus, it makes gfn_to_page_many_atomic
agnostic of multiple VCPU address spaces.

Reviewed-by: Radim Krcmar <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>