3469 dbuf_read_impl shows too much enthusiasm
Reviewed by: Dan McDonald <danmcd@omniti.com>

*** 639,650 ****
                  ASSERT3U(bonuslen, <=, db->db.db_size);
                  db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                  arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                  if (bonuslen < DN_MAX_BONUSLEN)
                          bzero(db->db.db_data, DN_MAX_BONUSLEN);
!                 if (bonuslen)
!                         bcopy(DN_BONUS(dn->dn_phys), db->db.db_data, bonuslen);
                  DB_DNODE_EXIT(db);
                  db->db_state = DB_CACHED;
                  mutex_exit(&db->db_mtx);
                  return;
          }
--- 639,667 ----
                  ASSERT3U(bonuslen, <=, db->db.db_size);
                  db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                  arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                  if (bonuslen < DN_MAX_BONUSLEN)
                          bzero(db->db.db_data, DN_MAX_BONUSLEN);
!                 if (bonuslen) {
!                         /*
!                          * Absent byzantine on-disk corruption, we fully expect
!                          * our bonuslen to be no more than DN_MAX_BONUSLEN --
!                          * but we nonetheless explicitly clamp it on the
!                          * bcopy() to prevent any on-disk corruption from
!                          * becoming rampant in-kernel corruption.
!                          */
!                         if (bonuslen > DN_MAX_BONUSLEN) {
!                                 DTRACE_PROBE3(dbuf__read__impl__toolong, int,
!                                     bonuslen, dnode_t *, dn, dmu_buf_impl_t *,
!                                     db);
!                                 bcopy(DN_BONUS(dn->dn_phys), db->db.db_data,
!                                     DN_MAX_BONUSLEN);
!                         } else {
!                                 bcopy(DN_BONUS(dn->dn_phys), db->db.db_data,
!                                     bonuslen);
!                         }
!                 }
                  DB_DNODE_EXIT(db);
                  db->db_state = DB_CACHED;
                  mutex_exit(&db->db_mtx);
                  return;
          }