Multipath autoreplace, control enclosure LEDs, event rate limiting

1. Enable multipath autoreplace support for FMA.

This extends FMA autoreplace to work with multipath disks.  This
requires libdevmapper to be installed at build time.

2. Turn on/off fault LEDs when VDEVs become degraded/faulted/online

Set ZED_USE_ENCLOSURE_LEDS=1 in zed.rc to have ZED turn on/off the enclosure
LED for a drive when a drive becomes FAULTED/DEGRADED.  Your enclosure must
be supported by the Linux SES driver for this to work.  The enclosure LED
scripts work for multipath devices as well.  The scripts will clear the LED
when the fault is cleared.

3. Rate limit ZIO delay and checksum events so as not to flood ZED

ZIO delay and checksum events are rate limited to 5/sec in the zfs module.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed by: Don Brady <don.brady@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #2449 
Closes #3017 
Closes #5159

include/sys/vdev_impl.h

@@ -34,6 +34,7 @@
 #include <sys/vdev.h>
 #include <sys/dkio.h>
 #include <sys/uberblock_impl.h>
+#include <sys/zfs_ratelimit.h>
 
 #ifdef	__cplusplus
 extern "C" {
@@ -243,6 +244,15 @@ struct vdev {
 	kmutex_t	vdev_dtl_lock;	/* vdev_dtl_{map,resilver}	*/
 	kmutex_t	vdev_stat_lock;	/* vdev_stat			*/
 	kmutex_t	vdev_probe_lock; /* protects vdev_probe_zio	*/
+
+	/*
+	 * 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;
 };
 
 #define	VDEV_RAIDZ_MAXPARITY	3