Record skipped MMP writes in multihost_history

Once per pass through the MMP thread's loop, the vdev tree is walked to
find a suitable leaf to write the next MMP block to. If no such leaf is
found, the thread sleeps for a while and resumes at the top of the loop.

Add an entry to multihost_history when no leaf can be found, and record
the reason in the error column. The error code for such entries is a
bitfield, displayed in hex:

0x1 At least one vdev (interior or leaf) was not writeable.
0x2 At least one writeable leaf vdev was found, but it had a pending
MMP write.

timestamp = the time in seconds since the epoch when no leaf could be
found originally.

duration = the time (in ns) during which no MMP block was written for
this reason. This does not include the preceeding inter-write period
nor the following inter-write period.

vdev_guid = the number of sequential cycles of the MMP thread looop when
this occurred.

Sample output, truncated to fit:

For records of skipped MMP writes the right-most column, vdev_path, is
reported as "-".

id txg timestamp error duration mmp_delay vdev_guid ...
936 11 1520036441 0 146264 891422313 1740883117838 ...
937 11 1520036441 0 163956 888356657 7320395061548 ...
938 11 1520036442 0 130690 885314969 7320395061548 ...
939 11 1520036442 0 2001068577 882296582 1740883117838 ...
940 11 1520036443 0 161806 882296582 7320395061548 ...
941 11 1520036443 0x2 0 998020546 1 ...
942 11 1520036444 0 136585 998020546 7320395061548 ...
943 11 1520036444 0x2 0 998020257 1 ...
944 11 1520036445 5 2002662964 994160219 1740883117838 ...
945 11 1520036445 0x2 998073118 994160219 3 ...
946 11 1520036447 0 247136 994160219 7320395061548 ...

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7212