o Enhance vm_pager_free_nonreq() function:

o Enhance vm_pager_free_nonreq() function:
  - Allow to call the function with vm object lock held.
  - Allow to specify reqpage that doesn't match any page in the region,
    meaning freeing all pages.
o Utilize the new function in couple more places in vnode pager.

Reviewed by:	alc, kib
Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.