6495 Fix mutex leak in dmu_objset_find_dp

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Albert Lee <trisk@omniti.com>
Author: Steven Hartland <steven.hartland@multiplay.co.uk>

illumos/illumos-gate@2bad22584defe4667f99737e3158d336e4dcca11