Some metadata arrays are pretty big (e.g. `EthernetTypeMetadata` has
65536 slots), while only a small subset of the values are filled with
actual decoders (e.g. for `EthernetTypeMetadata` that is 18 entries).

The rest were filled in `func init()` to return error on access. This
leads to noticeable startup time delay, as these mappings are pretty
big.

This change simply leaves not used slots unset (zero value), and the
request error values are generated on access to the slot.

Comparing time with `GODEBUG=inittrace=1` on a simple test program which
imports `gopacket/layers`:

before:

```
init internal/bytealg @0 ms, 0 ms clock, 0 bytes, 0 allocs
init runtime @0.009 ms, 0.063 ms clock, 0 bytes, 0 allocs
init math @0.18 ms, 0 ms clock, 0 bytes, 0 allocs
init errors @0.19 ms, 0 ms clock, 0 bytes, 0 allocs
init sync @0.20 ms, 0.002 ms clock, 16 bytes, 1 allocs
init internal/godebug @0.21 ms, 0.020 ms clock, 816 bytes, 20 allocs
init internal/intern @0.23 ms, 0.002 ms clock, 408 bytes, 8 allocs
init hash/crc32 @0.24 ms, 0.008 ms clock, 1024 bytes, 1 allocs
init net/netip @0.26 ms, 0 ms clock, 48 bytes, 2 allocs
init syscall @0.26 ms, 0.003 ms clock, 640 bytes, 3 allocs
init time @0.27 ms, 0.002 ms clock, 0 bytes, 0 allocs
init context @0.28 ms, 0 ms clock, 96 bytes, 1 allocs
init io/fs @0.29 ms, 0 ms clock, 0 bytes, 0 allocs
init os @0.30 ms, 0.006 ms clock, 328 bytes, 7 allocs
init unicode @0.31 ms, 0 ms clock, 512 bytes, 4 allocs
init reflect @0.32 ms, 0 ms clock, 0 bytes, 0 allocs
init vendor/golang.org/x/net/dns/dnsmessage @0.32 ms, 0.002 ms clock, 624 bytes, 6 allocs
init net @0.33 ms, 0.002 ms clock, 688 bytes, 13 allocs
init github.com/gopacket/gopacket @0.34 ms, 0.003 ms clock, 720 bytes, 5 allocs
init github.com/gopacket/gopacket/layers @0.35 ms, 1.3 ms clock, 25136 bytes, 63 allocs
```

after:

```
init internal/bytealg @0 ms, 0 ms clock, 0 bytes, 0 allocs
init runtime @0.011 ms, 0.070 ms clock, 0 bytes, 0 allocs
init math @0.26 ms, 0 ms clock, 0 bytes, 0 allocs
init errors @0.27 ms, 0 ms clock, 0 bytes, 0 allocs
init sync @0.27 ms, 0.003 ms clock, 16 bytes, 1 allocs
init internal/godebug @0.28 ms, 0.023 ms clock, 816 bytes, 20 allocs
init internal/intern @0.31 ms, 0.002 ms clock, 408 bytes, 8 allocs
init hash/crc32 @0.32 ms, 0.008 ms clock, 1024 bytes, 1 allocs
init net/netip @0.33 ms, 0 ms clock, 48 bytes, 2 allocs
init syscall @0.34 ms, 0.003 ms clock, 640 bytes, 3 allocs
init time @0.35 ms, 0.003 ms clock, 0 bytes, 0 allocs
init context @0.37 ms, 0 ms clock, 96 bytes, 1 allocs
init io/fs @0.38 ms, 0 ms clock, 0 bytes, 0 allocs
init os @0.38 ms, 0.009 ms clock, 328 bytes, 7 allocs
init unicode @0.40 ms, 0 ms clock, 512 bytes, 4 allocs
init reflect @0.41 ms, 0 ms clock, 0 bytes, 0 allocs
init vendor/golang.org/x/net/dns/dnsmessage @0.42 ms, 0.002 ms clock, 624 bytes, 6 allocs
init net @0.43 ms, 0.004 ms clock, 688 bytes, 13 allocs
init github.com/gopacket/gopacket @0.44 ms, 0.003 ms clock, 720 bytes, 5 allocs
init github.com/gopacket/gopacket/layers @0.46 ms, 0.086 ms clock, 25872 bytes, 66 allocs
```

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>