geli: add a read-only kern.geom.eli.use_uma_bytes sysctl

It reports the value of the g_eli_alloc_sz variable. Allocations of
this size or less will use UMA. Larger allocations will use malloc.
Since malloc is slower, it is useful for users to know this variable so
they can avoid such allocations. For example, ZFS users can set
vfs.zfs.vdev.aggregation_limit to this value.

MFC after: 1 week
Sponsored by: Axcient
Reviewed by: markj, imp
Differential Revision: https://reviews.freebsd.org/D44904