sfxge: consolidate privilege check functions

To reduce code duplication in common code, consolidate similar privilege
check functions.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4480