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/

Change-Id: Ib668f8fdb62ca01fe14321c07bd14d218744d909
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>