aio: hold an extra file reference over AIO read/write operations

Otherwise we might dereference an already freed file and/or inode
when aio_complete is called before we return from the read_iter or
write_iter method.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit 0b944d3a4bba6b25f43aed530f4fa85c04d162a6)
Change-Id: I628a87b5036ba1ba5ba5152fa0329d02999d3649
Git-Commit: 0b944d3a4bba6b25f43aed530f4fa85c04d162a6
Git-Repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


[riteshh@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>