fix: optimize DropReason eBPF map lookups

[rectified95]

Description

For all eBPF program in the DropReason plugin (except inet_csk_accept which has issue we need to investigate #715):

• only make ebpf map calls when necessary
• omit setting some packet fields to 0 right after memset is called on the entire struct

Details:
Previously, we did a map lookup regardless of whether the input retVal indicated a drop. Now, only for drops.
We also skip a map delete when there wasn't a earlier kprobe that saved the corresponding PID.

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