NFSD: Fix licensing header in filecache.c

Add a missing SPDX header.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/nfsd/filecache.c

+// SPDX-License-Identifier: GPL-2.0
 /*
- * Open file cache.
+ * The NFSD open file cache.
  *
  * (c) 2015 - Jeff Layton <jeff.layton@primarydata.com>
  *