autofs4 - fix get_next_positive_dentry()

The initialization condition in fs/autofs4/expire.c:get_next_positive_dentry()
appears to be incorrect. If prev == NULL I believe that root should be
returned.

Further down, at the current dentry check for it being simple_positive()
it looks like the d_lock for dentry p should be dropped instead of dentry
ret, otherwise when p is assinged to ret we end up with no lock on p and
a lost lock on ret, which leads to a deadlock.
Signed-off-by: NIan Kent <raven@themaw.net>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/autofs4/expire.c

@@ -96,7 +96,7 @@ static struct dentry *get_next_positive_dentry(struct dentry *prev,
 	struct dentry *p, *ret;
 
 	if (prev == NULL)
-		return dget(prev);
+		return dget(root);
 
 	spin_lock(&autofs4_lock);
 relock:
@@ -133,7 +133,7 @@ static struct dentry *get_next_positive_dentry(struct dentry *prev,
 	spin_lock_nested(&ret->d_lock, DENTRY_D_LOCK_NESTED);
 	/* Negative dentry - try next */
 	if (!simple_positive(ret)) {
-		spin_unlock(&ret->d_lock);
+		spin_unlock(&p->d_lock);
 		p = ret;
 		goto again;
 	}