2976 remove useless offsetof() macros

*** 27,36 ****
--- 27,37 ----
  
  #include <sys/dklabel.h>
  #include <sys/dktp/fdisk.h>
  #include <stdlib.h>
  #include <stdarg.h>
+ #include <stddef.h>
  #include <unistd.h>
  #include <strings.h>
  #include <errno.h>
  #include <limits.h>
  
*** 145,157 ****
  /*
   * Introduce an index into the list to speed up insert_into looking for the
   * right position in the list. This index is an AVL tree of all the
   * iodev_snapshot in the list.
   */
- 
- #define offsetof(s, m)  (size_t)(&(((s *)0)->m))        /* for avl_create */
- 
  static int
  avl_iodev_cmp(const void* is1, const void* is2)
  {
          int c = iodev_cmp((struct iodev_snapshot *)is1,
              (struct iodev_snapshot *)is2);
--- 146,155 ----