ovl: decode pure upper file handles

Decoding an upper file handle is done by decoding the upper dentry from
underlying upper fs, finding or allocating an overlay inode that is
hashed by the real upper inode and instantiating an overlay dentry with
that inode.

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