What parts of UMA are part of the stable ABI?

[Ryan Stone]

I've put the full patch to convert uma_malloc and uma_free to accept a
vm_size_t up for review[1].  It ended up being more extensive than expected
as a fair number of places do use uma_set_allocf().  I do plan on MFC'ing
this patch.  This survive a make tinderbox (ignoring some vt-related LINT
kernel build failures)

I haven't attempted converting the uma ctors, etc over to vm_size_t yet,
but I do plan on doing that still.

[1] https://reviews.freebsd.org/D2106