[POWERPC] Check _PAGE_RW and _PAGE_PRESENT on kernel addresses

Previously, the TLB miss handlers assumed that pages above KERNELBASE are
always present and read/write.  This assumption is false in the case of
CONFIG_DEBUG_PAGEALLOC.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>