MFC r269963+269964:

MFC r269963+269964:

Re-instate UMA cached backend for 4K - 64K allocations.  New consumers
like geli(4) uses malloc(9) to allocate temporary buffers that gets
free'ed shortly, causing frequent TLB shootdown as observed in hwpmc
supported flame graph.

Add a new loader tunable, vm.kmem_zmax which allows a system administrator
to limit the maximum allocation size that malloc(9) would consider using
the UMA cache allocator as backend.

git-svn-id: svn://svn.freebsd.org/base/stable/10@270811 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f