Use IPFW_RULE_CNTR_SIZE macro instead of non-relevant ip_fw_cntr structure.

Found by:	luigi


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273260 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Remove an unused mutex.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273261 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f