ovl: lookup non-dir copy-up-origin by file handle

If overlay.origin xattr is found on a non-dir upper inode try to get lower
dentry by calling exportfs_decode_fh().

On failure to lookup by file handle to lower layer, do not lookup the copy
up origin by name, because the lower found by name could be another file in
case the upper file was renamed.

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