[POWERPC] update_mmu_cache: Don't cache-flush non-readable pages

Currently, update_mmu_cache will crash if given a no-access PTE.
There's no need to synchronize dcache/icache unless it's an exec
mapping -- however, due to the existence of older glibc versions that
execute out of a read-but-no-exec page, readability is tested instead.

This assumes no exec-only mappings; if such mappings become supported,
they will need to go through the kmap_atomic() version of
dcache/icache synchronization.

This fixes a bug reported by some users where the kernel would crash
while dumping core on a threaded program.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>