const-ify make segment ops structures
There is no reason to keep the segment ops structures writable.

*** 54,65 ****
  #include <vm/seg_dev.h>
  #include <vm/seg_vn.h>
  #include <vm/seg_spt.h>
  #include <vm/seg_kmem.h>
  
! extern struct seg_ops segdev_ops;       /* needs a header file */
! extern struct seg_ops segspt_shmops;    /* needs a header file */
  
  static int
  page_valid(struct seg *seg, caddr_t addr)
  {
          struct segvn_data *svd;
--- 54,65 ----
  #include <vm/seg_dev.h>
  #include <vm/seg_vn.h>
  #include <vm/seg_spt.h>
  #include <vm/seg_kmem.h>
  
! extern const struct seg_ops segdev_ops; /* needs a header file */
! extern const struct seg_ops segspt_shmops;      /* needs a header file */
  
  static int
  page_valid(struct seg *seg, caddr_t addr)
  {
          struct segvn_data *svd;