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.

Change-Id: Iab923235308ff57348ffc2dc6d718dd64040656b
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>