cachefiles: use path_get instead of lone dget

Dentry references should not be acquired without a corresponding
vfsmount ref.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>