the sx lock was used previously because we might sleep allocating
additional memory by using auto-extending sbufs. However, we no longer
do this, instead retaining the user-submitted rule string, so mutexes
can be used instead. Annotate the reason for not using the sbuf-related
rule-to-string code with a comment.
Switch to using TAILQ_CONCAT() instead of manual list copying, as it's
O(1), reducing the rule replacement step under the mutex from O(2N) to
O(2).
Remove now uneeded vnode-related includes.
MFC after: 2 weeks
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