[POWERPC] Use PAGE_OFFSET to tell if an address is user/kernel in SW TLB handlers

Move to using PAGE_OFFSET instead of TASK_SIZE or KERNELBASE value on
6xx/40x/44x/fsl-booke to determine if the faulting address is a kernel or
user space address.  This mimics how the macro is_kernel_addr() works.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>