Illumos 5117 - spacemap reallocation can cause corruption

5117 space map reallocation can cause corruption
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Richard Elling <richard.elling@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:

https://www.illumos.org/projects/illumos-gate/issues/5117
https://github.com/illumos/illumos-gate/commit/e503a68

Ported by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2662