Linux 3.16: Switch to iter_file_splice_write

Users of generic_file_splice_write need to switch to
using iter_file_splice_write.

Reviewed-on: http://gerrit.openafs.org/11302
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 e284db57f94c8f97ed1c95dcd0bd9518d86c050c)

Change-Id: I6f88ec0388fe43accc150c7243c1a474c9e643af
Reviewed-on: http://gerrit.openafs.org/11308
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>