Provide emulation of asynchronous operation in swap_pager.c itself, not
in the vm_pager.h inliner.

Suggested by:	kib
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Make the iodone callback responsible on unbusying the reqpage.

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