Volatile ranges with fallocate()

I can't speak for Google, but I have a patch I'll be sending out in my next iteration that removes the ashmem unpinned range management code and replaces it with fallocate() calls (killing ~320 lines in ashmem).

Ashmem also serves a separate purpose to Android, which is to provide a way to create atomically unlinked tmpfs files that can be shared between applications. This avoids having applications accidentally leave tmpfs files that take memory even if on one is using them.

For this purpose, the ashmem driver still is useful, but can be submitted and reviewed independently without tangling the pin/unpin functionality into the discussion.