Fix access check

Fixes a bug where USE_CPUSET used access(F_OK)
as a sign that it should try writing to the cpuset
path. It needs to instead use access(W_OK) to
ensure that it has write access as that's what
it would try to open() with.

And stop trying to repeatedly open a file that
doesn't exist.

Change-Id: Ib391ee6f08345051c48a986a732eda30143cb614