tcp: fix list iteration in tcp_lro_flush_active()

Use LIST_FOREACH_SAFE(), since the list element is removed from
the list in the loop body, zero out and inserted in the free list.

Reviewed by: rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46383

(cherry picked from commit 64443828bbe7c571db8d8731758ec8c4b8364c86)