Previous injection assumes the next instruction to `printk()` is
"goto <label_return>", which isn't guaranteed by clang.

Technically, previous injection adds a ret insn to implement "exit
immediately if not matched":

    asm.Return().WithSymbol("return")

which forces us to remove the duplicated assumed "goto <label_return>"
after `printk()`.

This commit improves the way to inject ebpf pcap filter without such
assumption by setting proper values to registers to avoid adding ret
insn.

Signed-off-by: Zhichuan Liang <gray.liang@isovalent.com>

`kfree_skbmem` marks the end of an skb's lifetime, so we stop
tracking it at that time when `--filter-track-skb` is enabled.

Signed-off-by: Zhichuan Liang <gray.liang@isovalent.com>