locking/rwsem: Remove the list_head from struct rw_semaphore

locking/rwsem: Remove the list_head from struct rw_semaphore

Instead of embedding a list_head in struct rw_semaphore, store a pointer
to the first waiter.  The list of waiters remains a doubly linked list
so we can efficiently add to the tail of the list, remove from the front
(or middle) of the list.

Some of the list manipulation becomes more complicated, but it's a
reasonable tradeoff on the slow paths to shrink some core data structures
like struct inode.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260305195545.3707590-2-willy@infradead.org