bpf: sync <kdir>/include/.../bpf.h with tools/include/.../bpf.h

This patch copies changes in bpf.h done by a previous patch
in this patchset from the kernel uapi include dir into tools
uapi include dir.

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>