CFR: Fix panic in zfs_umount() due to dropped root vnode refcount

[Will Andrews]

Hi,

Diff:
http://people.freebsd.org/~will/zfs/zfs_vfsops.c-fix-unmount-panic.diff

Change 660985 by willa at willa_SpectraBSD on 2013/03/08 17:43:49

        Fix the zfs unmount panic that is due to a missed refcount drop.

        When checking for activity in a non-forced unmount, remember that
        we called vflush() releasing the refcount that zfs_mount() placed
        on the filesystem's root vnode using VFS_ROOT, and restore it.

        Without this change, the next time around, the missing refcount
        led to a second release of the root vnode when its refcount was
        already zero, and thus the panic occurred while trying to grab
        the vnode's interlock, which had been destroyed when the vnode's
        refcount dropped to zero.

        While I'm here, refactor the unmount failure code to ensure that
        the zfs ctldir (i.e. .zfs/.snapshots) is always restored, along
with,
        if needed, zfsvfs->z_unmounted = B_FALSE.  In short, if the unmount
        fails, restore all original functionality prior to unmounting.

This particular panic manifests easily if you have active NFS I/O on a ZFS
while trying to do a non-forced unmount.  We've also seen it happen on
shutdown occasionally.

I would like to commit this or a version of it, to FreeBSD/head next week.

Thanks,
--Will.