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>