[RFC PATCH] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM

[Chaitanya Kulkarni]

> @Chaitanya:
> 
> Since this is a mere simplification and shorten of code, I suppose I can skip
> the performance tests. Ira and I have still hundreds of call sites with kmap()
> and kmap_atomic() which we should care of, therefore we prefer to leave alone
> everything that is not strictly necessary for the deprecated API deletions.
> 
> Thanks to you all,
> 

fine by me.

-ck