quoted

I'd think that it's treated as a nomral string in quite a few places, 

so

quoted

it needs to be NUL terminated. (But not searched the sources.)

Why not?  It isn't hard to find.

fs/dcache.c:703:        str[name->len] = 0;

That's technically an area where it's treated like a normal string, but 
not really relevant to the question (which is "what if I don't 
nul-terminate the thing"?  Do you have an example handy of a strcmp() or 
such that would fail if the NUL terminator were not there?