powerpc/mem: Blacklist flush_dcache_icache_phys() for kprobe

kprobe does not handle events happening in real mode, all
functions running with MMU disabled have to be blacklisted.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eaab3bff961c3bfe149f1d0bd3593291ef939dcc.1585670437.git.christophe.leroy@c-s.fr