Make nfs_file_cred more robust.

As not all files have an associated open_context (e.g. device special
files), it is safest to test for the existence of the open context
before de-referencing it.

Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/nfs4proc.c

···
 		struct nfs_open_context *ctx;
 
 		ctx = nfs_file_open_context(sattr->ia_file);
-		cred = ctx->cred;
-		state = ctx->state;
+		if (ctx) {
+			cred = ctx->cred;
+			state = ctx->state;
+		}
 	}
 
 	status = nfs4_do_setattr(inode, cred, fattr, sattr, state);

include/linux/nfs_fs.h

···
 
 static inline struct rpc_cred *nfs_file_cred(struct file *file)
 {
-	if (file != NULL)
-		return nfs_file_open_context(file)->cred;
+	if (file != NULL) {
+		struct nfs_open_context *ctx =
+			nfs_file_open_context(file);
+		if (ctx)
+			return ctx->cred;
+	}
 	return NULL;
 }