powerpc/mm/hash: Reduce contention on hpte lock

We do this in some part. This patch make sure we always try to search
for hpte without holding lock and redo the compare with lock held once
match found.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>