ceph: fix improper use of smp_mb__before_atomic()

This barrier only applies to the read-modify-write operations; in
particular, it does not apply to the atomic64_set() primitive.

Replace the barrier with an smp_mb().

Fixes: fdd4e158 ("ceph: rework dcache readdir")
Reported-by: N"Paul E. McKenney" <paulmck@linux.ibm.com>
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NAndrea Parri <andrea.parri@amarulasolutions.com>
Reviewed-by: N"Yan, Zheng" <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/super.h

@@ -545,7 +545,12 @@ static inline void __ceph_dir_set_complete(struct ceph_inode_info *ci,
 					   long long release_count,
 					   long long ordered_count)
 {
-	smp_mb__before_atomic();
+	/*
+	 * Makes sure operations that setup readdir cache (update page
+	 * cache and i_size) are strongly ordered w.r.t. the following
+	 * atomic64_set() operations.
+	 */
+	smp_mb();
 	atomic64_set(&ci->i_complete_seq[0], release_count);
 	atomic64_set(&ci->i_complete_seq[1], ordered_count);
 }