Do not munmap dead pages.

This causes perf problems since we spend a lot of time munmapping pages.
The code already madvises away pages that are not in use.

Bug: 17524229

(cherry pick from commit ef19c12ffa052a654e6fb9b51be26923804950c6)

Change-Id: I1ba6952994fc82f46b4e4a6772ac0a842164f057