FreeBSD: Update use of UMA-related symbols in arc_available_memory

Recent UMA changes 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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Bojan Novković <bnovkov@FreeBSD.org>
Closes #16230