keys: Fix the use of the C++ keyword "private" in uapi/linux/keyctl.h

The keyctl_dh_params struct in uapi/linux/keyctl.h contains the symbol
"private" which means that the header file will cause compilation failure
if #included in to a C++ program.  Further, the patch that added the same
struct to the keyutils package named the symbol "priv", not "private".

The previous attempt to fix this (commit 8a2336e5) did so by simply
renaming the kernel's copy of the field to dh_private, but this then breaks
existing userspace and as such has been reverted (commit 8c0f9f5b).

[And note, to those who think that wrapping the struct in extern "C" {}
 will work: it won't; that only changes how symbol names are presented to
 the assembler and linker.].

Instead, insert an anonymous union around the "private" member and add a
second member in there with the name "priv" to match the one in the
keyutils package.  The "private" member is then wrapped in !__cplusplus
cpp-conditionals to hide it from C++.

Fixes: ddbb4114 ("KEYS: Add KEYCTL_DH_COMPUTE command")
Fixes: 8a2336e5

 ("uapi/linux/keyctl.h: don't use C++ reserved keyword as a struct member name")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Randy Dunlap <rdunlap@infradead.org>
cc: Lubomir Rintel <lkundrak@v3.sk>
cc: James Morris <jmorris@namei.org>
cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
cc: Stephan Mueller <smueller@chronox.de>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.morris@microsoft.com>