fs: use a consume fence in mnt_idmap()

The routine is used in link_path_walk() for every path component.

To my reading the entire point of the fence was to grab a fully
populated mnt_idmap, but that's already going to happen with mere
consume fence.

Eliminates an actual fence on arm64.

Signed-off-by: Mateusz Guzik <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: mjguzik

include/linux/mount.h

@@ -76,7 +76,7 @@ struct vfsmount {
 static inline struct mnt_idmap *mnt_idmap(const struct vfsmount *mnt)
 {
 	/* Pairs with smp_store_release() in do_idmap_mount(). */
-	return smp_load_acquire(&mnt->mnt_idmap);
+	return READ_ONCE(mnt->mnt_idmap);
 }
 
 extern int mnt_want_write(struct vfsmount *mnt);