OpenZFS 9880 - Race in ZFS parallel mount

OpenZFS 9880 - Race in ZFS parallel mount

Porting Notes:
* Not required for Linux since the zone is always global.  But
  we'll want this change if we start using the zones code.

Authored by: Andy Fiddaman <omnios@citrus-it.co.uk>
Reviewed by: Jason King <jason.king@joyent.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Approved by: Joshua M. Clulow <josh@sysmgr.org>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/9880
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/bc4c0ff134
Closes #8189