LinuxKPI: add Exponentially Weighted Moving Average implementation

Add DECLARE_EWMA() which expands to a per-name EWMA implementation
as used by multiple wireless drivers.

Sposnored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: hselasky, cperciva, dwmalone
Differential Revision: https://reviews.freebsd.org/D30415