Fixing the mount_smbfs(8) hanging by utilising the destroy_dev_sched() KPI.

Relevant threads:

http://lists.freebsd.org/pipermail/freebsd-current/2007-June/074329.html

Reviewed by: kib, bp (slightly different version)
Tested by: Yuri Pankov <yuri.pankov at gmail dot com>,

		Jiawei Ye <leafy7382 at gmail dot com>

Approved by: re (kensmith)