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 <[e-mail]>
Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-dust.c

···
 	.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);
 
···
 	return result;
 }
 
-void __exit dm_dust_exit(void)
+static void __exit dm_dust_exit(void)
 {
 	dm_unregister_target(&dust_target);
 }