zfs: Update use of UMA-related symbols in arc_available_memory

da76d34 repurposed the use of UMA_MD_SMALL_ALLOC in a way that breaks
arc_available_memory on -CURRENT. This change ensures that
arc_available_memory uses the new symbol while maintaining compatibility
with older FreeBSD releases. This change was submitted to upstream
as well.

Approved by: markj (mentor)
Fixes: da76d34