dm: don't report "detected capacity change" on device creation

When a DM device is first created it doesn't yet have an established
capacity, therefore the use of set_capacity_and_notify() should be
conditional given the potential for needless pr_info "detected
capacity change" noise even if capacity is 0.

One could argue that the pr_info() in set_capacity_and_notify() is
misplaced, but that position is not held uniformly.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: f64d9b2e

 ("dm: use set_capacity_and_notify")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>