Re-add kstat.zfs.misc.arcstats.other_size under COMPAT_FREEBSD11.

It is used by a number of applications, notably top(1).

Reported by: netchild
Reviewed by: allanjude
Approved by: re (delphij)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16943