In the case of forced unmount of the lower filesystem, the lower
mount may be destroyed while nullfs is still mounted above it.
Further, since struct mount is type-stable, the lower mount may
also be recycled for a different FS while the nullfs instance
still holds a pointer to it.
Instead, always always access the lower mount through the v_mount
field of the lower root vnode. Since nullfs holds a reference to that
vnode, it will not be recycled to a different mount and its v_mount
field will always be either the valid lower mount or NULL.
After obtaining the lower mount, attempt to busy it and handle failure