bpf: verifier: hard wire branches to dead code

Loading programs with dead code becomes more and more
common, as people begin to patch constants at load time.
Turn conditional jumps to unconditional ones, to avoid
potential branch misprediction penalty.

This optimization is enabled for privileged users only.

For branches which just fall through we could just mark
them as not seen and have dead code removal take care of
them, but that seems less clean.

v0.2:
 - don't call capable(CAP_SYS_ADMIN) twice (Jiong).
v3:
 - fix GCC warning;

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>