powerpc: add barrier after writing kernel PTE

There is no barrier between something like ioremap() writing to
a PTE, and returning the value to a caller that may then store the
pointer in a place that is visible to other CPUs.  Such callers
generally don't perform barriers of their own.

Even if callers of ioremap() and similar things did use barriers,
the most logical choise would be smp_wmb(), which is not
architecturally sufficient when BookE hardware tablewalk is used.  A
full sync is specified by the architecture.

For userspace mappings, OTOH, we generally already have an lwsync due
to locking, and if we occasionally take a spurious fault due to not
having a full sync with hardware tablewalk, it will not be fatal
because we will retry rather than oops.

Signed-off-by: Scott Wood <scottwood@freescale.com>