xattr: add listxattr helper

Add a tiny helper to determine whether an xattr handler given a specific
dentry supports listing the requested xattr. We will use this helper in
various filesystems in later commits.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

include/linux/xattr.h

@@ -47,6 +47,22 @@ struct xattr_handler {
 		   size_t size, int flags);
 };
 
+/**
+ * xattr_handler_can_list - check whether xattr can be listed
+ * @handler: handler for this type of xattr
+ * @dentry: dentry whose inode xattr to list
+ *
+ * Determine whether the xattr associated with @dentry can be listed given
+ * @handler.
+ *
+ * Return: true if xattr can be listed, false if not.
+ */
+static inline bool xattr_handler_can_list(const struct xattr_handler *handler,
+					  struct dentry *dentry)
+{
+	return handler && (!handler->list || handler->list(dentry));
+}
+
 const char *xattr_full_name(const struct xattr_handler *, const char *);
 
 struct xattr {