pf: fix panic on deferred packets

The pfsync_defer_tmo() callout needs to set the correct vnet before it
can transmit packets. It used the rcvif in the mbuf to get this vnet,
but that doesn't work for locally originated traffic. In that case the
rcvif pointer is NULL, and the dereference leads to a panic.

Instead use the sc_sync_if, which is always set (if pfsync is enabled,
at least).

PR: 268246
MFC after: 2 weeks