Linux 3.16: Switch to iter_file_splice_write

Users of generic_file_splice_write need to switch to
using iter_file_splice_write.

Change-Id: If4801d27e030e1cb986f483cf437a2cfa7398eb3
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>