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>