extensions: libxt_bpf: Fix build with old kernel versions

extensions: libxt_bpf: Fix build with old kernel versions

In kernel 3.18 the union bpf_attr does not have a pathname attribute and
BPF_OBJ_GET is also not defined in these versions.
This was added in Linux commit b2197755b263 ("bpf: add support for
persistent maps/progs"). Check for the BPF_FS_MAGIC define which was
also added in this Linux commit and only activate this code in case we
find that define.

This fixes a build problem with Linux 3.18.
Netfilter bug: #1231

Fixes: f17f9ace8a8 ("extensions: libxt_bpf: support ebpf pinned objects")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>