namei: add NDPREINIT() macro
Its intent is to do the initialization of the future part of struct nameidata
which should be used across several namei() and VOPs. Right now it is NOP.
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