LU-13846 llite: move iov iter forward by ourself

LU-13846 llite: move iov iter forward by ourself

Newer kernel will reward iov iter back to original
position for direct IO, see following codes:

        iov_iter_revert(from, write_len -
                        iov_iter_count(from));--------->here
out:
        return written;
}
EXPORT_SYMBOL(generic_file_direct_write);

This break assumptions from Lustre and caused problem
when Lustre need split one IO to several io loop, considering
4M block IO for 1 MiB stripe file, it will submit first 1MiB IO
4 times and caused data corruptions finally.

Since generic kernel varies from different kernel versions,
we'd better fix this problem by move iov iter forward by
Lustre itself.

Added a new test cases aiocp.c is copied from xfstests,
with codes style cleanups to make checkpatch.pl happy.

Change-Id: Iab5d8f1bb0e74ed49c821c81b734c68770edf4a8
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/39565
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>