Evaluate using memfd_secret() for Linux >= 5.14

[cypherbits]

Linux 5.14 has the new memfd_secret syscall, it would be good to use it.
More info: https://lwn.net/Articles/865256/

[jedisct1]

Can you clarify when, how and why that system call would be useful?

Now that the uncached mappings flag has been removed, it doesn't seem to be very useful any more.