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

@@ -128,11 +128,10 @@
         .getoffset      = nop,
         .gettype        = nop,
         .getvp          = nop,
         .advise         = nop,
         .setpagesize    = nop,
-        .getmemid       = nop,
         .getpolicy      = nop,
 #endif
 };
 
 /*