Illumos 5176 - lock contention on godfather zio

5176 lock contention on godfather zio
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Elling <richard.elling@gmail.com>
Reviewed by: Bayard Bell <Bayard.Bell@nexenta.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  https://www.illumos.org/issues/5176
  https://github.com/illumos/illumos-gate/commit/6f834bc

Porting notes:

Under Linux max_ncpus is defined as num_possible_cpus().  This is
largest number of cpu ids which might be available during the life
time of the system boot.  This value can be larger than the number
of present cpus if CONFIG_HOTPLUG_CPU is defined.

Ported by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2711