This CL adds in the necessary types and constants to support Linux
filesystem encryption. This includes the structs for the keys and
polices as well as the constants for key size and encryption algorithms.

This code also reduces the scope of some of the regexes in mkerrors.sh.
This is to reduce the number of unrelated constants added in with this
change.

Note that due to a bug in the Linux uapi headers, not all of the
necessary constants and structures are declared in linux/fs.h despite
being part of the kernel ABI. This means that some constants and types
had to be manually added in.

The bug has been patched (https://patchwork.kernel.org/patch/9662723),
and the manual additions can be removed when the patch is merged into
the mainline kernel.

Change-Id: Ib508ad99bdf4c0068933ffcf351c52bb359cfcf4
Reviewed-on: https://go-review.googlesource.com/41417Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>