KVM: PPC: Book3S HV: Add helpers for lock/unlock hpte

This adds helper routines for locking and unlocking HPTEs, and uses
them in the rest of the code.  We don't change any locking rules in
this patch.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>