LU-6260 llite: add support for new iter functionality

For the 3.16+ kernels struct file_operations added new read
and write methods; read_iter and write_iter; to use struct
iov_iter as a parameter since it contains all the iovec
data needed. This avoid having the file system managing
iovec data like transversing the iovec page list. Now we
can use supplied iov_iter helper functions that does this
work for us. In later kernels only this back end is
supported. Backported from the upstream lustre client.

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Linux-commit: b42b15fdad3ebb790250041d1517acebb9bd56d9

lustre: get rid of messing with iovecs

* switch to ->read_iter/->write_iter
* keep a pointer to iov_iter instead of iov/nr_segs
* do not modify iovecs; use iov_iter_truncate()/iov_iter_advance() and
  a new primitive - iov_iter_reexpand() (expand previously truncated
  iterator) istead.
* (racy) check for lustre VMAs intersecting with iovecs kept for now as
  for_each_iov() loop.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Change-Id: I71928beaa7b1c87f3e2c689e1dee96052eaef872
Reviewed-on: http://review.whamcloud.com/15028
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>