Call nvlist_free before return

Fixes a small kernel memory leak which would occur if a pool failed
to import because the DMU_POOL_VDEV_ZAP_MAP key can't be read from
a presumably damaged MOS config. In the case of a missing key there
was no leak.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Finix1979 <yancw@info2soft.com>
Closes #13629