7254 ztest failed assertion in ztest_dataset_dirobj_verify: dirobjs + 1 == usedobjs

dsl_dataset_space is looking at the ds_bp's fill count while
dmu_objset_write_ready() is concurrently modifying it. This fix adds an
rrwlock to protect the ds_bp.

Closes #180

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Author: Paul Dagnelie <pcd@delphix.com>