netdevsim: dev: Fix memory leak in nsim_dev_take_snapshot_write

In case memory resources for dummy_data were allocated, release them
before return.

Addresses-Coverity-ID: 1491997 ("Resource leak")
Fixes: 7ef19d3b

 ("devlink: report error once U32_MAX snapshot ids have been used")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>