autofs: fix use-after-free in lockless ->d_manage()

autofs_d_release() can overlap with lockless ->d_manage(),
ending up with autofs_dentry_ino() freed under the latter.
Make freeing autofs_info instances RCU-delayed...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/autofs/autofs_i.h

@@ -71,6 +71,7 @@ struct autofs_info {
 
 	kuid_t uid;
 	kgid_t gid;
+	struct rcu_head rcu;
 };
 
 #define AUTOFS_INF_EXPIRING	(1<<0) /* dentry in the process of expiring */

fs/autofs/inode.c

@@ -36,7 +36,7 @@ void autofs_clean_ino(struct autofs_info *ino)
 
 void autofs_free_ino(struct autofs_info *ino)
 {
-	kfree(ino);
+	kfree_rcu(ino, rcu);
 }
 
 void autofs_kill_sb(struct super_block *sb)