Fix 'prevsnap property' build failure

Fix build failure accidentally introduced by 1715493. This only
results in a failure when debugging is disabled.

dsl_dataset.c: In function 'dsl_dataset_stats':
dsl_dataset.c:1698:45: error: 'dp' undeclared (first use in this function)

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>