4047 panic from dbuf_free_range() from dmu_free_object() while doing zfs receive
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

*** 613,622 ****
--- 613,623 ----
           * any level-0 dbufs that will be completely freed.  Therefore we must
           * exercise care to not read or count the first and last blocks
           * if they are blocksize-aligned.
           */
          if (dn->dn_datablkshift == 0) {
+                 if (off != 0 || len < dn->dn_datablksz)
                          dmu_tx_count_write(txh, off, len);
          } else {
                  /* first block will be modified if it is not aligned */
                  if (!IS_P2ALIGNED(off, 1 << dn->dn_datablkshift))
                          dmu_tx_count_write(txh, off, 1);