Do not initiate MMP writes while pool is suspended

While the pool is suspended on host A, it may be imported on host B.
If host A continued to write MMP blocks, it would be blindly
overwriting MMP blocks written by host B, and the blocks written by
host A would have outdated txg information.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7182

module/zfs/mmp.c

@@ -437,7 +437,7 @@ mmp_thread(spa_t *spa)
 			zio_suspend(spa, NULL);
 		}
 
-		if (multihost)
+		if (multihost && !suspended)
 			mmp_write_uberblock(spa);
 
 		CALLB_CPR_SAFE_BEGIN(&cpr);