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

@@ -54,12 +54,12 @@
 #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 */
+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;