uma: Fix improper uses of UMA_MD_SMALL_ALLOC

UMA_MD_SMALL_ALLOC was recently replaced by UMA_USE_DMAP, but
da76d349b6b1 missed some improper uses of the old symbol.
This change makes sure that UMA_USE_DMAP is used properly in
code that selects uma_small_alloc.

Fixes: da76d349b6b1
Reported by: eduardo, rlibby
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D45368