powerpc/kprobes: Move kprobes over to patch_instruction()

arch_arm/disarm_probe() use direct assignment for copying
instructions, replace them with patch_instruction(). We don't need to
call flush_icache_range() because patch_instruction() does it for us.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>