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

[Al Viro]

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?