Accessing an mbuf after it has been handed off to the hardware is a bad race as it could already have been tx'd and freed by that time.  Place the bpf tap just _before_ writing the gen bit.
Accessing an mbuf after it has been handed off to the hardware is a bad
race as it could already have been tx'd and freed by that time.  Place
the bpf tap just _before_ writing the gen bit.

This fixes a panic when running tcpdump on a cxgb interface.

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