When we set OFFPAGE to limit fragmentation we should also set VTOSLAB
so that we avoid the hashtables. The hashtable is now only required if
a zone is created with OFFPAGE specified initially, not internally. This
flag signals to UMA that it can't touch the allocated memory and so
can't store a slab pointer in the containing page.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D22453