third_party/ashmem: Fixup ashmem_get_prot_region for memfd

mmap with MAP_PRIVATE is meant to succeed even on write-protected
memfd's as it will just copy-on-write.

Bug: 1296216, 1299437
Change-Id: Iaed4d0e873f777e6fdc96d9215b28165d48b423f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3465975


Reviewed-by: Egor Pasko <pasko@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975730}