Unclear usage of NM_OPEN_NO_MMAP

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Doing a search through the repo for NM_OPEN_NO_MMAP I can't find where it is 
actually used. In fact the effect it's supposed to have is determined entirely 
by nr_arg2 in the nm_req structure. As long as the parent desc has the same 
nr_arg2 req.nr_arg2, even 0, the mmap address is used.

This should be revised, it's in the public API and it's confusing.

Original issue reported on code.google.com by ciprian....@linaro.org on 11 Nov 2014 at 1:27