The sys_quotactl() contract demands that the mount point is
vfs_unbusy()ed when the cmd is Q_QUOTAON, regardless of other input
parameters or error return.

Submitted by: Conrad Meyer
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D1684
Tested by: pho
MFC after: 1 week