Linux 3.16: Convert to new write_iter/read_iter ops

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

Change-Id: Ia58e5f90182f3968d1a81cfc2f831e7a9cf35d93
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>