FreeBSD: incorporate changes to the VFS_QUOTACTL(9) KPI

VFS_QUOTACTL(9) has been updated to allow each filesystem to indicate
whether it has changed the busy state of the mount. The filesystem
may still assume that its .vfs_quotactl entrypoint is always called
with the mount busied, but only needs to unbusy the mount (and clear
*mp_busy) if it does something that actually requires the mount to be
unbusied. It no longer needs to blindly copy-paste the UFS protocol
for calling vfs_unbusy(9) for the Q_QUOTAOFF and Q_QUOTAON commands.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Jason Harmening <jason.harmening@gmail.com>
Closes #12052