Linux: Test for __vfs_write rather than __vfs_read

Linux: Test for __vfs_write rather than __vfs_read

The following commit:

    commit eb031849d52e61d24ba54e9d27553189ff328174
    Author: Christoph Hellwig <hch@lst.de>
    Date:   Fri Sep 1 17:39:23 2017 +0200

        fs: unexport __vfs_read/__vfs_write

unexports both __vfs_read and __vfs_write, but keeps the former in
fs.h--as it is is still being used by another part of the tree.

This situation results in a false positive in our Autoconf check,
which does not see the export statements, and ends up marking the
corresponding API as available.

That, in turn, causes some code which assumes symmetry with
__vfs_write to fail to compile.

Switch to testing for __vfs_write, which correctly marks the API as
unavailable.

Reviewed-on: https://gerrit.openafs.org/12728
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 929e77a886fc9853ee292ba1aa52a920c454e94b)

Change-Id: I03e3c8222360a6b04b45b45a8f56b5df054f6783
Reviewed-on: https://gerrit.openafs.org/12736
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>