MFV r322223: 8378 crash due to bp in-memory modification of nopwrite block

MFV r322223: 8378 crash due to bp in-memory modification of nopwrite block

illumos/illumos-gate@b7edcb940884114e61382937505433c4c38c0278
https://github.com/illumos/illumos-gate/commit/b7edcb940884114e61382937505433c4c38c0278

https://www.illumos.org/issues/8378
  The problem is that zfs_get_data() supplies a stale zgd_bp to dmu_sync(), which
  we then nopwrite against.
  zfs_get_data() doesn't hold any DMU-related locks, so after it copies db_blkptr
  to zgd_bp, dbuf_write_ready()
  could change db_blkptr, and dbuf_write_done() could remove the dirty record.
  dmu_sync() then sees the stale
  BP and that the dbuf it not dirty, so it is eligible for nop-writing.
  The fix is for dmu_sync() to copy db_blkptr to zgd_bp after acquiring the
  db_mtx. We could still see a stale
  db_blkptr, but if it is stale then the dirty record will still exist and thus
  we won't attempt to nopwrite.

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>

MFC after:	2 weeks