powerpc/mm: Blacklist SLB symbols from kprobe

We can't sensibly take a trap at this point. So, blacklist these
symbols.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>