ppc/hugetlbfs: Replace ACCESS_ONCE with READ_ONCE

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

)

Change the ppc/hugetlbfs code to replace ACCESS_ONCE with READ_ONCE.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>