Silence compile-time warning on unused variable

ASSERT3U() could be NOP which then leads to having unused pointer *spa.

metaslab.c: In function 'metaslab_condense':
metaslab.c:2075:9: warning: unused variable 'spa' [-Wunused-variable]

  spa_t *spa = msp->ms_group->mg_vd->vdev_spa;

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7489