tools/bpf: Sync uapi header bpf.h

sync uapi header include/uapi/linux/bpf.h to
tools/include/uapi/linux/bpf.h

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200115184308.162644-7-brianvv@google.com