Illumos 4976-4984 - metaslab improvements

4976 zfs should only avoid writing to a failing non-redundant top-level vdev
4978 ztest fails in get_metaslab_refcount()
4979 extend free space histogram to device and pool
4980 metaslabs should have a fragmentation metric
4981 remove fragmented ops vector from block allocator
4982 space_map object should proactively upgrade when feature is enabled
4983 need to collect metaslab information via mdb
4984 device selection should use fragmentation metric
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <adam.leventhal@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:

https://www.illumos.org/issues/4976
https://www.illumos.org/issues/4978
https://www.illumos.org/issues/4979
https://www.illumos.org/issues/4980
https://www.illumos.org/issues/4981
https://www.illumos.org/issues/4982
https://www.illumos.org/issues/4983
https://www.illumos.org/issues/4984
https://github.com/illumos/illumos-gate/commit/2e4c998

Notes:

The "zdb -M" option has been re-tasked to display the new metaslab
fragmentation metric and the new "zdb -I" option is used to control
the maximum number of in-flight I/Os.

The new fragmentation metric is derived from the space map histogram
which has been rolled up to the vdev and pool level and is presented
to the user via "zpool list".

Add a number of module parameters related to the new metaslab weighting
logic.

Ported by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2595