call vm_lowmem hook in uma_reclaim_worker

A comment near kmem_reclaim() implies that we already did that.
Calling the hook is useful, because some handlers, e.g. ARC,
might be able to release significant amounts of KVA.

Now that we have more than one place where vm_lowmem hook is called,
use this change as an opportunity to introduce flags that describe
a reason for calling the hook. No handler makes use of the flags yet.

Reviewed by: markj, kib
MFC after: 1 week
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D9764