vfs: stop counting on gcc not messing with mnt_expiry_mark if not asked

So happens it already was not doing it, but there is no need to "hope"
as indicated in the comment.

No changes in generated assembly.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20231004111916.728135-3-mjguzik@gmail.comSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -1346,9 +1346,9 @@ void mntput(struct vfsmount *mnt)
 {
 	if (mnt) {
 		struct mount *m = real_mount(mnt);
-		/* avoid cacheline pingpong, hope gcc doesn't get "smart" */
+		/* avoid cacheline pingpong */
 		if (unlikely(m->mnt_expiry_mark))
-			m->mnt_expiry_mark = 0;
+			WRITE_ONCE(m->mnt_expiry_mark, 0);
 		mntput_no_expire(m);
 	}
 }