Linux: Use kernel_read/kernel_write when __vfs variants are unavailable

Linux: Use kernel_read/kernel_write when __vfs variants are unavailable

We hide the uses of set_fs/get_fs behind a macro, as those functions
are likely to soon become unavailable:

> Christoph Hellwig suggested removing all calls outside of the core
> filesystem and architecture code; Andy Lutomirski went one step
> further and said they should all go.

    https://lwn.net/Articles/722267/

Reviewed-on: https://gerrit.openafs.org/12729
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 5ee516b3789d3545f3d78fb3aba2480308359945)

Change-Id: I28a7126bf6ab048f8d949f190e557a3fa44f3f46
Reviewed-on: https://gerrit.openafs.org/12737
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>