tools lib api fs: Add bpf_fs filesystem detector

Allow mounting of the BPF filesystem at /sys/fs/bpf.

Signed-off-by: Joe Stringer <joe@ovn.org>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: netdev@vger.kernel.org
Link: http://lkml.kernel.org/r/20170126212001.14103-6-joe@ovn.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>