filter: refactor common filter attach code into __sk_attach_prog

Both sk_attach_filter() and sk_attach_bpf() are setting up sk_filter,
charging skmem and attaching it to the socket after we got the eBPF
prog up and ready. Lets refactor that into a common helper.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>