Turn d_set_d_op() BUG_ON() into WARN_ON_ONCE()

It's indicative of a real problem, and it actually triggers with
autofs4, but the BUG_ON() is excessive.  The autofs4 case is being fixed
(to only set d_op in the ->lookup method) but not merged yet.  In the
meantime this gets the code limping along.

Reported-by: Alex Elder <aelder@sgi.com>
Cc: Ian Kent <raven@themaw.net>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -1357,8 +1357,8 @@ EXPORT_SYMBOL(d_alloc_name);
 
 void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 {
-	BUG_ON(dentry->d_op);
-	BUG_ON(dentry->d_flags & (DCACHE_OP_HASH	|
+	WARN_ON_ONCE(dentry->d_op);
+	WARN_ON_ONCE(dentry->d_flags & (DCACHE_OP_HASH	|
 				DCACHE_OP_COMPARE	|
 				DCACHE_OP_REVALIDATE	|
 				DCACHE_OP_DELETE ));