Do not hold the sysctl lock across a call to the handler. This fixes a
general LOR issue where the sysctl lock had no good place in the
hierarchy. One specific instance is #284 on
http://sources.zabbadoz.net/freebsd/lor.html .

Reviewed by: jhb
MFC after: 1 month
X-MFC-note: split oid_refcnt field for oid_running to preserve KBI