vfs: Update lookup_dcache() comment

commit 6c51e513 ("lookup_dcache(): lift d_alloc() into callers")
removed the need_lookup argument from lookup_dcache(), but the
comment was forgotten. Also it no longer allocates a new dentry
if nothing was found.
Signed-off-by: NOleg Drokin <green@linuxhacker.ru>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1449,9 +1449,8 @@ static int follow_dotdot(struct nameidata *nd)
 }
 
 /*
- * This looks up the name in dcache, possibly revalidates the old dentry and
- * allocates a new one if not found or not valid.  In the need_lookup argument
- * returns whether i_op->lookup is necessary.
+ * This looks up the name in dcache and possibly revalidates the found dentry.
+ * NULL is returned if the dentry does not exist in the cache.
  */
 static struct dentry *lookup_dcache(const struct qstr *name,
 				    struct dentry *dir,