vfs: add NDREINIT to facilitate repeated namei calls
struct nameidata mixes caller arguments, internal state and output, which
can be quite error prone.
Recent addition of valdiating ni_resflags uncovered a caller which could
repeatedly call namei, effectively operating on partially populated state.
Add bare minimium validation this does not happen. The real fix would
decouple aforementioned state.
Reported by: pho
Tested by: pho (different variant)