Cleanup: zvol_add_clones() should not NULL check dp

It is never NULL because we return early if dsl_pool_hold() fails.

This caused Coverity to complain.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14042