Change checksum & IO delay ratelimit values

Change checksum & IO delay ratelimit thresholds from 5/sec to 20/sec.
This allows zed to actually trigger if a bunch of these events arrive in
a short period of time (zed has a threshold of 10 events in 10 sec).
Previously, if you had, say, 100 checksum errors in 1 sec, it would get
ratelimited to 5/sec which wouldn't trigger zed to fault the drive.

Also, convert the checksum and IO delay thresholds to module params for
easy testing.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7252

include/sys/vdev_impl.h

@@ -262,8 +262,6 @@ struct vdev {
 	 * We rate limit ZIO delay and ZIO checksum events, since they
 	 * can flood ZED with tons of events when a drive is acting up.
 	 */
-#define	DELAYS_PER_SECOND 5
-#define	CHECKSUMS_PER_SECOND 5
 	zfs_ratelimit_t vdev_delay_rl;
 	zfs_ratelimit_t vdev_checksum_rl;
 };

include/sys/zfs_ratelimit.h

@@ -25,13 +25,19 @@
 typedef struct {
 	hrtime_t start;
 	unsigned int count;
-	unsigned int burst;		/* Number to allow per interval */
-	unsigned int interval;		/* Interval length in seconds */
+
+	/*
+	 * Pointer to number of events per interval.  We do this to
+	 * allow the burst to be a (changeable) module parameter.
+	 */
+	unsigned int *burst;
+
+	unsigned int interval;	/* Interval length in seconds */
 	kmutex_t lock;
 } zfs_ratelimit_t;
 
 int zfs_ratelimit(zfs_ratelimit_t *rl);
-void zfs_ratelimit_init(zfs_ratelimit_t *rl, unsigned int burst,
+void zfs_ratelimit_init(zfs_ratelimit_t *rl, unsigned int *burst,
     unsigned int interval);
 void zfs_ratelimit_fini(zfs_ratelimit_t *rl);