iflib: null out freed mbuf in iflib_txsd_free

When adding the IFLIB_GET_MBUF/FLAGS, I neglected to NULL out the
mbuf in the descriptor ring. I didn't think this should matter as
the I thought this code was only used when the ring was about
to be freed. But I was wrong, and leaving a stale mbuf in there can
cause panics.

Reported by: Marek Zarychta (https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292547)
Fixes: 14d93f612f26
Sponsored by: Netflix