Add Module Parameter Regarding Log Size Limit

* Add Module Parameters Regarding Log Size Limit

zfs_wrlog_data_max
The upper limit of TX_WRITE log data. Once it is reached,
write operation is blocked, until log data is cleared out
after txg sync. It only counts TX_WRITE log with WR_COPIED
or WR_NEED_COPY.

Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: jxdking <lostking2008@hotmail.com>
Closes #12284

module/zfs/arc.c

@@ -7980,6 +7980,18 @@ arc_init(void)
 		zfs_dirty_data_max = MIN(zfs_dirty_data_max,
 		    zfs_dirty_data_max_max);
 	}
+
+	if (zfs_wrlog_data_max == 0) {
+
+		/*
+		 * dp_wrlog_total is reduced for each txg at the end of
+		 * spa_sync(). However, dp_dirty_total is reduced every time
+		 * a block is written out. Thus under normal operation,
+		 * dp_wrlog_total could grow 2 times as big as
+		 * zfs_dirty_data_max.
+		 */
+		zfs_wrlog_data_max = zfs_dirty_data_max * 2;
+	}
 }
 
 void