ebpf: remove CONFIG_BPF_SYSCALL ifdefs in socket filter code

This gets rid of CONFIG_BPF_SYSCALL ifdefs in the socket filter code,
now that the BPF internal header can deal with it.

While going over it, I also changed eBPF related functions to a sk_filter
prefix to be more consistent with the rest of the file.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>