MFC r349979:

Recycle the unused FR_CMPSIZ macro which became orphaned in ipfilter 5
prior to its import into FreeBSD. This macro calculates the size to be
compared within the frentry structure. The ipfilter 4 version of the
macro calculated the compare size based upon the static size of the
frentry struct. Today it uses the ipfilter 5 method of calculating the
size based upon the new to ipfilter 5 fr_size value found in the
frentry struct itself.

No effective change in code is intended.