OpenZFS 7181 - race between zfs_mount and zfs_ioc_rollback

Authored by: Andriy Gapon <andriy.gapon@clusterhq.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>
Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7181
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/90f2c09
Closes #5585