x/sys/unix: add support for new fscrypt ioctls from Linux v5.4

[ebiggers]

For google/fscrypt#148, we need the sys/unix package to support the latest fscrypt UAPI from Linux v5.4. This includes new ioctls and some other updates.

I'm working on a patch for this.

[gopherbot]

Change https://golang.org/cl/208898 mentions this issue: unix/linux: add new fscrypt declarations from Linux v5.4

[dmitshur]

/cc @ianlancetaylor @bradfitz @tklauser per owners.