ovl: fix IOCB_DIRECT if underlying fs doesn't support direct IO

Normally the check at open time suffices, but e.g loop device does set
IOCB_DIRECT after doing its own checks (which are not sufficent for
overlayfs).

Make sure we don't call the underlying filesystem read/write method with
the IOCB_DIRECT if it's not supported.

Reported-by: Huang Jianan <huangjianan@oppo.com>
Fixes: 16914e6f ("ovl: add ovl_read_iter()")
Cc: <stable@vger.kernel.org> # v4.19
Tested-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>