fs/namespace.c: bury long-dead define

MNT_WRITER_UNDERFLOW_LIMIT has been missed 4 years ago when it became unused.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -63,8 +63,6 @@ static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
 	return tmp & (HASH_SIZE - 1);
 }
 
-#define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16)
-
 /*
  * allocation is serialized by namespace_sem, but we need the spinlock to
  * serialize with freeing.