Use vm_page_replace_checked() instead of vm_page_rename() for implementing
optimized copy-on-write faults. This has two advantages: (1) one less radix
tree operation is performed and (2) vm_page_replace_checked() cannot fail,
making the code simpler.

Submitted by: Ryan Libby
Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4478