Linux 3.16: Convert to new write_iter/read_iter ops

Change read/write operations to the new write_iter/read_iter
operations.

Reviewed-on: http://gerrit.openafs.org/11303
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit a303bb257ed9e790d8c14644779e9508167887b6)

Change-Id: I3f66104be067698a4724ed78537765cf26d4aa10
Reviewed-on: http://gerrit.openafs.org/11309
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>