OpenZFS 7086 - ztest attempts dva_get_dsize_sync on an embedded blockpointer

In dbuf_dirty(), we need to grab the dn_struct_rwlock before looking at
the db_blkptr, to prevent it from being changed by syncing context.

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7086
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/98fa317
Closes #5039