dm: change kzalloc to kcalloc

Use 2-factor multiplication argument form kcalloc() instead
of instead of the deprecated kzalloc() [1].

[1] https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
Link: https://github.com/KSPP/linux/issues/162



Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>