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.

*** 2012,2022 ****
  }
  
  int
  segop_getmemid(struct seg *seg, caddr_t addr, memid_t *mp)
  {
!         VERIFY3P(seg->s_ops->getmemid, !=, NULL);
  
          return (seg->s_ops->getmemid(seg, addr, mp));
  }
  
  struct lgrp_mem_policy_info *
--- 2012,2023 ----
  }
  
  int
  segop_getmemid(struct seg *seg, caddr_t addr, memid_t *mp)
  {
!         if (seg->s_ops->getmemid == NULL)
!                 return (ENODEV);
  
          return (seg->s_ops->getmemid(seg, addr, mp));
  }
  
  struct lgrp_mem_policy_info *