Illumos 5164-5165 - space map fixes

5164 space_map_max_blksz causes panic, does not work
5165 zdb fails assertion when run on pool with recently-enabled

space map_histogram feature

Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/5164
https://www.illumos.org/issues/5165
https://github.com/illumos/illumos-gate/commit/b1be289

Porting Notes:

The metaslab_fragmentation() hunk was dropped from this patch
because it was already resolved by commit 8b0a084.

The comment modified in metaslab.c was updated to use the correct
variable name, space_map_blksz. The upstream commit incorrectly
used space_map_blksize.

Ported by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2697