dm persistent data: remove unused return from exit_shadow_spine()

Fix the following coccicheck warnings:

./drivers/md/persistent-data/dm-btree-spine.c:188:5-6: Unneeded
variable: "r". Return "0" on line 194.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/persistent-data/dm-btree-internal.h

@@ -83,7 +83,7 @@ struct shadow_spine {
 };
 
 void init_shadow_spine(struct shadow_spine *s, struct dm_btree_info *info);
-int exit_shadow_spine(struct shadow_spine *s);
+void exit_shadow_spine(struct shadow_spine *s);
 
 int shadow_step(struct shadow_spine *s, dm_block_t b,
 		struct dm_btree_value_type *vt);

drivers/md/persistent-data/dm-btree-spine.c

@@ -183,15 +183,13 @@ void init_shadow_spine(struct shadow_spine *s, struct dm_btree_info *info)
 	s->count = 0;
 }
 
-int exit_shadow_spine(struct shadow_spine *s)
+void exit_shadow_spine(struct shadow_spine *s)
 {
-	int r = 0, i;
+	int i;
 
 	for (i = 0; i < s->count; i++) {
 		unlock_block(s->info, s->nodes[i]);
 	}
-
-	return r;
 }
 
 int shadow_step(struct shadow_spine *s, dm_block_t b,