dm: Remove unnecessary return in dm_zone_endio()

The return statement at the end of dm_zone_endio() is not needed.
Remove it.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>