ANDROID: GKI: Enable CONFIG_USERFAULTFD

Patches for SELinux support and kernel page-fault restriction in
userfaultfd have been backported. See references below.
So from security perspective it should be safe to enable it in Android.

1) https://android-review.googlesource.com/c/kernel/common/+/1576486
2) https://android-review.googlesource.com/c/kernel/common/+/1576704
3) https://android-review.googlesource.com/c/kernel/common/+/1612597
4) https://android-review.googlesource.com/c/kernel/common/+/1574667



Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Bug: 160737021
Bug: 169683130
Change-Id: Iac5143da76783de57dba229f5761aff9297c17ae