Add a WARN_ON_ONCE() to check for passing an unreferenced dentry to dput().

This is analogous to the similar check in dget(), and will make
reference-counting bugs in filesystems more immediately obvious. (I
just spent a while debugging an oops that turned out to be due to
broken fs reference counting.)
Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/dcache.c

@@ -217,6 +217,7 @@ void dput(struct dentry *dentry)
 	if (!dentry)
 		return;
 
+	WARN_ON_ONCE(!atomic_read(&dentry->d_count));
 repeat:
 	if (atomic_read(&dentry->d_count) == 1)
 		might_sleep();