net: use jump_label for netstamp_needed

netstamp_needed seems a good candidate to jump_label conversion.

This avoids 3 conditional branches per incoming packet in fast path.

No measurable difference, given that these conditional branches are
predicted on modern cpus. Only a small icache reduction, thanks to the
unlikely() stuff.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>