ZoL #1886, Illumos #4322

4322 ZFS deadlock on dp_config_rwlock
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Ilya Usvyatsky <ilya.usvyatsky@nexenta.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  https://www.illumos.org/issues/4322
  https://github.com/illumos/illumos-gate/commit/c50d56f667f119d78fa3d94d6bef2c298ba556f6

Ported by: Chris Dunlop <chris@onthe.net.au>
Closes #1886