md/raid5-cache: Annotate pslot with __rcu notation

radix_tree_lookup_slot() and radix_tree_replace_slot() API expect the
slot returned and looked up to be marked with __rcu. Otherwise
sparse warnings are generated:

  drivers/md/raid5-cache.c:2939:23: warning: incorrect type in
			assignment (different address spaces)
  drivers/md/raid5-cache.c:2939:23:    expected void **pslot
  drivers/md/raid5-cache.c:2939:23:    got void [noderef] __rcu **
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/raid5-cache.c

@@ -2637,7 +2637,7 @@ int r5c_try_caching_write(struct r5conf *conf,
 	int i;
 	struct r5dev *dev;
 	int to_cache = 0;
-	void **pslot;
+	void __rcu **pslot;
 	sector_t tree_index;
 	int ret;
 	uintptr_t refcount;
@@ -2804,7 +2804,7 @@ void r5c_finish_stripe_write_out(struct r5conf *conf,
 	int i;
 	int do_wakeup = 0;
 	sector_t tree_index;
-	void **pslot;
+	void __rcu **pslot;
 	uintptr_t refcount;
 
 	if (!log || !test_bit(R5_InJournal, &sh->dev[sh->pd_idx].flags))