fs: failure to unmount on cleanup is a fatal error

Any subsequent attempt to remove the mountdir will fail if the unmount
does not succeed.

Use syscall.Rmdir rather than os.Remove. We know the mount point is a
directory, and Go's cleverness can only complicate matters here.

Change-Id: I62cfad63f34af17b78c3184884a2972673f631a3

fs/mem_test.go

@@ -34,9 +34,9 @@ func testMount(t *testing.T, root InodeEmbedder, opts *Options) (string, *fuse.S
 	}
 	return mntDir, server, func() {
 		if err := server.Unmount(); err != nil {
-			t.Errorf("testMount: Unmount failed: %v", err)
+			t.Fatalf("testMount: Unmount failed: %v", err)
 		}
-		if err := os.Remove(mntDir); err != nil {
+		if err := syscall.Rmdir(mntDir); err != nil {
 			t.Errorf("testMount: Remove failed: %v", err)
 		}
 	}