namei: Fix cn_flags width in 9pfs

This truncation is mostly harmless today, but fix it anyway to avoid
pain later down the road.

Reviewed by: olce, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50417

(partially cherry picked from commit 0d224af399a66f00a5b33e5512fc018062cabf1d)