__dentry_path() fixes

* we need to save the starting point for restarts
* reject pathologically short buffers outright

Spotted-by: Denys Vlasenko <dvlasenk@redhat.com>
Spotted-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -3116,19 +3116,22 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 /*
  * Write full pathname from the root of the filesystem into the buffer.
  */
-static char *__dentry_path(struct dentry *dentry, char *buf, int buflen)
+static char *__dentry_path(struct dentry *d, char *buf, int buflen)
 {
+	struct dentry *dentry;
 	char *end, *retval;
 	int len, seq = 0;
 	int error = 0;
 
+	if (buflen < 2)
+		goto Elong;
+
 	rcu_read_lock();
 restart:
+	dentry = d;
 	end = buf + buflen;
 	len = buflen;
 	prepend(&end, &len, "\0", 1);
-	if (buflen < 1)
-		goto Elong;
 	/* Get '/' right */
 	retval = end-1;
 	*retval = '/';