vm_fault: Fix some nits in vm_fault_copy_entry()

• Correct the description (vm_fault_copy_entry() does not create a shadow object).
• Move some initialization and assertions out of the scope of the object locks, when doing so makes sense.
• Merge a pair of conditional blocks.
• Use __unused when appropriate.

No functional change intended.

Reviewed by: alc
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b57be759d079d40aa6ec75207faadce5ae1484a2)