dm dust: Make dm_dust_init and dm_dust_exit static

Fix sparse warnings:

drivers/md/dm-dust.c:495:12: warning: symbol 'dm_dust_init' was not declared. Should it be static?
drivers/md/dm-dust.c:505:13: warning: symbol 'dm_dust_exit' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-dust.c

@@ -492,7 +492,7 @@ static struct target_type dust_target = {
 	.prepare_ioctl = dust_prepare_ioctl,
 };
 
-int __init dm_dust_init(void)
+static int __init dm_dust_init(void)
 {
 	int result = dm_register_target(&dust_target);
 
@@ -502,7 +502,7 @@ int __init dm_dust_init(void)
 	return result;
 }
 
-void __exit dm_dust_exit(void)
+static void __exit dm_dust_exit(void)
 {
 	dm_unregister_target(&dust_target);
 }