spa.c: Replace VERIFY(nvlist_*(...) == 0) with fnvlist_* (#12678)

The fnvlist versions of the functions are fatal if they fail,
saving each call from having to include checking the result.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Allan Jude <allan@klarasystems.com>