ovl: plumb through flush method

Filesystems can implement their own flush method that release
resources, or manipulate caches. Currently if one of these
filesystems is used with overlayfs, the flush method is not called.

[Amir: fix fd leak in ovl_flush()]

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>