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

@@ -228,11 +228,11 @@
 } svntr_stats_t;
 
 extern void     segvn_init(void);
 extern int      segvn_create(struct seg *, void *);
 
-extern  struct seg_ops segvn_ops;
+extern const struct seg_ops segvn_ops;
 
 /*
  * Provided as shorthand for creating user zfod segments.
  */
 extern  caddr_t zfod_argsp;