linuxkpi: Add list_is_first function

This function just test if the element is the first of the list.

Reviewed by:	hselasky
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D23766

sys/compat/linuxkpi/common/include/linux/list.h

@@ -449,6 +449,13 @@ static inline void list_cut_position(struct list_head *list,
 		__list_cut_position(list, head, entry);
 }
 
+static inline int list_is_first(const struct list_head *list,
+				const struct list_head *head)
+{
+
+	return (list->prev == head);
+}
+
 static inline int list_is_last(const struct list_head *list,
 				const struct list_head *head)
 {