net/packet: speed up packet decoding

The compiler is failing to draw the connection between
slice cap and slice len, so is missing some obvious BCE opportunities.
Give it a hint by making the cap equal to the length.
The generated code is smaller and cleaner, and a bit faster.

name              old time/op    new time/op    delta
Decode/tcp4-8       12.2ns ± 1%    11.6ns ± 3%  -5.31%  (p=0.000 n=28+29)
Decode/tcp6-8       12.5ns ± 2%    11.9ns ± 2%  -4.84%  (p=0.000 n=30+30)
Decode/udp4-8       11.5ns ± 1%    11.1ns ± 1%  -3.11%  (p=0.000 n=25+24)
Decode/udp6-8       11.8ns ± 3%    11.4ns ± 1%  -3.08%  (p=0.000 n=30+26)
Decode/icmp4-8      11.0ns ± 3%    10.6ns ± 1%  -3.38%  (p=0.000 n=25+30)
Decode/icmp6-8      11.4ns ± 1%    11.1ns ± 2%  -2.29%  (p=0.000 n=27+30)
Decode/igmp-8       10.3ns ± 0%    10.0ns ± 1%  -3.26%  (p=0.000 n=19+23)
Decode/unknown-8    8.68ns ± 1%    8.38ns ± 1%  -3.55%  (p=0.000 n=28+29)

net/packet/packet.go

@@ -150,6 +150,7 @@ func (q *Parsed) decode4(b []byte) {
 		return
 	}
 	sub := b[q.subofs:]
+	sub = sub[:len(sub):len(sub)] // help the compiler do bounds check elimination
 
 	// We don't care much about IP fragmentation, except insofar as it's
 	// used for firewall bypass attacks. The trick is make the first
@@ -267,6 +268,7 @@ func (q *Parsed) decode6(b []byte) {
 	// dropped.
 	q.subofs = 40
 	sub := b[q.subofs:]
+	sub = sub[:len(sub):len(sub)] // help the compiler do bounds check elimination
 
 	switch q.IPProto {
 	case ICMPv6: