unionfs: add patch to revert to old pre-1.0 meta directory

Unionfs changed its meta directory from .unionfs to .unionfs-fuse with the
unionfs -> unionfs-fuse rename.  The rename later got reverted everywhere
but the meta directory, so now unionfs doesn't find the whiteout files from
older releases.

Revert back to the pre-1.0 behaviour to fix this.

Patch merged upstream:
https://github.com/rpodgorny/unionfs-fuse/commit/93c2f9bd8b058b28016e6db2421e5b38eac0606c



[Thomas: add reference to the upstream commit, now that the patch has
been merged.]

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>