ovl: fix incorrect fdput() on aio completion

ovl_{read,write}_iter() always call fdput(real) to put one or zero
refcounts of the real file, but for aio, whether it was submitted or not,
ovl_aio_put() also calls fdput(), which is not balanced.  This is only a
problem in the less common case when FDPUT_FPUT flag is set.

To fix the problem use get_file() to take file refcount and use fput()
instead of fdput() in ovl_aio_put().

Fixes: 2406a307 ("ovl: implement async IO routines")
Cc: <stable@vger.kernel.org> # v5.6
Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>