md/raid5: factor out code for changing size of stripe cache.

Separate the actual 'change' code from the sysfs interface
so that it can eventually be called internally.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 0f86f5e..cbdbc77 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -497,4 +497,5 @@
 {
 	return layout >= 8 && layout <= 10;
 }
+extern int raid5_set_cache_size(mddev_t *mddev, int size);
 #endif