MFV 316896

7580 ztest failure in dbuf_read_impl

illumos/illumos-gate@1a01181fdc809f40c64d5c6881ae3e4521a9d9c7
https://github.com/illumos/illumos-gate/commit/1a01181fdc809f40c64d5c6881ae3e4521a9d9c7

https://www.illumos.org/issues/7580

We need to prevent any reader whenever we're about the zero out all the
blkptrs. To do this we need to grab the dn_struct_rwlock as writer in
dbuf_write_children_ready and free_children just prior to calling bzero.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: George Wilson <george.wilson@delphix.com>