Report duration and error in mmp_history entries

After an MMP write completes, update the relevant mmp_history entry
with the time between submission and completion, and the error
status of the write.

[faaland1@toss3a zfs]$ cat /proc/spl/kstat/zfs/pool/multihost
39 0 0x01 100 8800 69147946270893 72723903122926
id       txg     timestamp  error  duration   mmp_delay    vdev_guid
10607    1166    1518985089 0      138301     637785455    4882...
10608    1166    1518985089 0      136154     635407747    1151...
10609    1166    1518985089 0      803618560  633048078    9740...
10610    1166    1518985090 0      144826     633048078    4882...
10611    1166    1518985090 0      164527     666187671    1151...

Where duration = gethrtime_in_done_fn - gethrtime_at_submission, and
error = zio->io_error.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7190

include/sys/mmp.h

@@ -42,6 +42,7 @@ typedef struct mmp_thread {
 	uint64_t	mmp_delay;	/* decaying avg ns between MMP writes */
 	uberblock_t	mmp_ub;		/* last ub written by sync */
 	zio_t		*mmp_zio_root;	/* root of mmp write zios */
+	uint64_t	mmp_kstat_id;	/* unique id for next MMP write kstat */
 } mmp_thread_t;