dentry_cmp(): use lockless_dereference() instead of smp_read_barrier_depends()

lockless_dereference() was added which can be used in place of
hard-coding smp_read_barrier_depends().
Signed-off-by: He Kuang <hekuang@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -226,10 +226,9 @@ static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char
 
 static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
 {
-	const unsigned char *cs;
 	/*
 	 * Be careful about RCU walk racing with rename:
-	 * use ACCESS_ONCE to fetch the name pointer.
+	 * use 'lockless_dereference' to fetch the name pointer.
 	 *
 	 * NOTE! Even if a rename will mean that the length
 	 * was not loaded atomically, we don't care. The
@@ -243,8 +242,8 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 	 * early because the data cannot match (there can
 	 * be no NUL in the ct/tcount data)
 	 */
-	cs = ACCESS_ONCE(dentry->d_name.name);
-	smp_read_barrier_depends();
+	const unsigned char *cs = lockless_dereference(dentry->d_name.name);
+
 	return dentry_string_cmp(cs, ct, tcount);
 }