Linux 4.1: Don't define or use ->write directly

Linux 4.1: Don't define or use ->write directly

We no longer have to define a ->write operation, and we can't
expect the underlying disk cache filesystem to have one.  Use
the new __vfs_read/write helpers that will select the operation
to use based on what's available for that particular filesystem.

Reviewed-on: http://gerrit.openafs.org/11849
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 5c1237432edf4600111845d175c92252430d5f76)

Change-Id: I21bca85637e07d0e03ef471896d0454eeef68a14
Reviewed-on: http://gerrit.openafs.org/11873
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>