vm_page_queue_free_mtx is a spin mutex, not a normal sleep mutex.

vm_page_queue_free_mtx is a spin mutex, not a normal sleep mutex.
I do not know why this didn't panic my box, but I have most certainly
been using it:
peter@overcee[3:14pm]~src/sys/i386/i386-110> sysctl -a | grep zero
vm.stats.misc.zero_page_count: 2235
vm.stats.misc.cnt_prezero: 638951
vm.idlezero_enable: 1
vm.idlezero_maxrun: 16

Submitted by:	Tor.Egge@cvsup.no.freebsd.org
Approved by:	Tor's patches are never wrong. :-)