pax: Clear arcn in each read function.

Instead of initializing individual fields to zero, clear the entire
struct prior to populating it.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: 0mp, markj
Differential Revision: https://reviews.freebsd.org/D46097

(cherry picked from commit f890020d43bf2070f12ffec37bcd9579f6fb1347)