MFC r316809:

Fix a use after free panic in ipfilter's fragment processing.
Memory is malloc'd, then a search for a match in the fragment table
is made and if the fragment matches, the wrong fragment table is
freed, causing a use after free panic. This commit fixes this.

A symptom of the problem is a kernel page fault in bcopy() called by
ipf_frag_lookup() at line 715 in ip_frag.c. Another symptom is a
kernel page fault in ipf_frag_delete() when called by ipf_frag_expire()
via ipf_slowtimer().