unionfs: do not use bare struct componentname
Allocate nameidata on stack and NDPREINIT() it, for compatibility with
assumptions from other filesystems' lookup code.
Reviewed by: mckusick
Discussed with: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D30041