NFS: set varaiable nfs_netfs_debug_id storage-class-specifier to static

smatch reports
fs/nfs/fscache.c:260:10: warning: symbol
  'nfs_netfs_debug_id' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/fscache.c

@@ -257,7 +257,7 @@ int nfs_netfs_readahead(struct readahead_control *ractl)
 	return 0;
 }
 
-atomic_t nfs_netfs_debug_id;
+static atomic_t nfs_netfs_debug_id;
 static int nfs_netfs_init_request(struct netfs_io_request *rreq, struct file *file)
 {
 	rreq->netfs_priv = get_nfs_open_context(nfs_file_open_context(file));