io_uring: optimise INIT_WQ_LIST

The invariant of io_wq_work_list is that it's empty IFF ->first is NULL,
so no need to initially set ->last. With now having more users of the
list it may play a role, i.e. used in each tw iteration and on every
completion flushing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c464ab5cab6e46a858c6d39c107e92b3b5291f13.1633373302.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -38,7 +38,6 @@ struct io_wq_work_list {
 #define wq_list_empty(list)	(READ_ONCE((list)->first) == NULL)
 #define INIT_WQ_LIST(list)	do {				\
 	(list)->first = NULL;					\
-	(list)->last = NULL;					\
 } while (0)
 
 static inline void wq_list_add_after(struct io_wq_work_node *node,