mmp should use a fixed tag for spa_config locks

mmp_write_uberblock() and mmp_write_done() should the same tag
for spa_config_locks.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sanjeev Bagewadi <sanjeev.bagewadi@gmail.com>
Closes #6530
Closes #7155