Annotated dprintf as printf-like

ZFS loves using %llu for uint64_t, but that requires a cast to not 
be noisy - which is even done in many, though not all, places.
Also a couple places used %u for uint64_t, which were promoted
to %llu. 

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12233

module/zfs/spa_misc.c

@@ -615,7 +615,7 @@ spa_deadman(void *arg)
 
 	zfs_dbgmsg("slow spa_sync: started %llu seconds ago, calls %llu",
 	    (gethrtime() - spa->spa_sync_starttime) / NANOSEC,
-	    ++spa->spa_deadman_calls);
+	    (u_longlong_t)++spa->spa_deadman_calls);
 	if (zfs_deadman_enabled)
 		vdev_deadman(spa->spa_root_vdev, FTAG);