powerpc/bpf: Use memset32() to pre-fill traps in BPF page(s)

Use the newly introduced memset32() to pre-fill BPF page(s) with trap
instructions.

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