remove the last set_fs() in common code, and remove it for x86 and powerpc v2

[Christoph Hellwig]

On Tue, Sep 01, 2020 at 06:25:12PM +0100, Al Viro wrote:
> On Tue, Sep 01, 2020 at 07:13:00PM +0200, Christophe Leroy wrote:
> 
> >     10.92%  dd       [kernel.kallsyms]  [k] iov_iter_zero
> 
> Interesting...  Could you get an instruction-level profile inside iov_iter_zero(),
> along with the disassembly of that sucker?

So the interesting thing here is with that none of these code paths
should have changed at all, and the biggest items on the profile look
the same modulo some minor reordering.