o Since pager bufs are never put on freelist, share the space in
  the structure in union.
o Add iodone callback pointer to the b_pager.
o In vnode_pager use new b_pager.pg_iodone to store iodone
  pointer, eliminating the getpages_data structure and need to
  allocate it.

Suggested by:	kib
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.