port to Linux 3.13 (linux-next)
Convert direct_IO to use iov_iter so it will work with Linux-next (and therefore Linux 3.13)

https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=1359825cb7354b938e3c2a83ae7fad6ea911354d
Merge pull request #31 from benpicco/master

port to Linux 3.13 (linux-next)