use NULL setpagesize segop as a shorthand for ENOTSUP
Instead of forcing every segment driver to implement a dummp function to
return (hopefully) ENOTSUP, handle NULL setpagesize segop function pointer
as "return ENOTSUP" shorthand.

@@ -127,11 +127,10 @@
         .getprot        = nop,
         .getoffset      = nop,
         .gettype        = nop,
         .getvp          = nop,
         .advise         = nop,
-        .setpagesize    = nop,
         .getpolicy      = nop,
 #endif
 };
 
 /*