Illumos 6495 - Fix mutex leak in dmu_objset_find_dp

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>

References:

https://www.illumos.org/issues/6495
https://github.com/illumos/illumos-gate/commit/2bad225

Ported-by: Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>