ffs: fix build with GEOM_LABEL and without FFS, e.g. MINIMAL
The root of vfs.ffs sysctl tree was declared in ffs_alloc.c. The
1111a44301da started to use the root in ffs_subr.c. However, ffs_subr.c
may be included in kernels that do not have FFS in their config. Such
kernel won't link after 1111a44301da.