vfs cache: fix a hang when bumping vnode limit too high

Overflow in cache_changesize would make the value flip to 0 and stay
there as 0 << 1 does not do anything.

Note callers limit the outcome to something below u_int.

Also note there entire vnode handling thing both in vfs layer as a whole
and this file can't decide whether to long, u_long or u_int.