Use proper tag for spa config refcounts in mmp_write_uberblock()

This allows the spa config refcounts to use tracking in debug builds
without triggering the "No such hold %p on refcount" panic.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #8326