9880 Race in ZFS parallel mount

illumos/illumos-gate@bc4c0ff1343a311cc24933908ac6c4455af09031

Reviewed by: Jason King <jason.king@joyent.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Approved by: Joshua M. Clulow <josh@sysmgr.org>
Author: Andy Fiddaman <omnios@citrus-it.co.uk>