bcache: Fix incompatible pointer type warning

The function pointer release in struct block_device_operations
should point to functions declared as void.

Sparse warnings:

drivers/md/bcache/super.c:656:27: warning:
	incorrect type in initializer (different base types)
	drivers/md/bcache/super.c:656:27:
	expected void ( *release )( ... )
	drivers/md/bcache/super.c:656:27:
	got int ( static [toplevel] *<noident> )( ... )

drivers/md/bcache/super.c:656:2: warning:
	initialization from incompatible pointer type [enabled by default]

drivers/md/bcache/super.c:656:2: warning:
	(near initialization for ‘bcache_ops.release’) [enabled by default]
Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>

drivers/md/bcache/super.c

@@ -634,11 +634,10 @@ static int open_dev(struct block_device *b, fmode_t mode)
 	return 0;
 }
 
-static int release_dev(struct gendisk *b, fmode_t mode)
+static void release_dev(struct gendisk *b, fmode_t mode)
 {
 	struct bcache_device *d = b->private_data;
 	closure_put(&d->cl);
-	return 0;
 }
 
 static int ioctl_dev(struct block_device *b, fmode_t mode,