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.