ovl: allocate anon bdev per unique lower fs

Instead of allocating an anonymous bdev per lower layer, allocate
one anonymous bdev per every unique lower fs that is different than
upper fs.

Every unique lower fs is assigned an fsid > 0 and the number of
unique lower fs are stored in ofs->numlowerfs.

The assigned fsid is stored in the lower layer struct and will be
used also for inode number multiplexing.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>