x86/pkeys: Allow configuration of init_pkru

As discussed in the previous patch, there is a reliability
benefit to allowing an init value for the Protection Keys Rights
User register (PKRU) which differs from what the XSAVE hardware
provides.

But, having PKRU be 0 (its init value) provides some nonzero
amount of optimization potential to the hardware.  It can, for
instance, skip writes to the XSAVE buffer when it knows that PKRU
is in its init state.

The cost of losing this optimization is approximately 100 cycles
per context switch for a workload which lightly using XSAVE
state (something not using AVX much).  The overhead comes from a
combinaation of actually manipulating PKRU and the overhead of
pullin in an extra cacheline.

This overhead is not huge, but it's also not something that I
think we should unconditionally inflict on everyone.  So, make it
configurable both at boot-time and from debugfs.

Changes to the debugfs value affect all processes created after
the write to debugfs.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: Dave Hansen <dave@sr71.net>
Cc: mgorman@techsingularity.net
Cc: arnd@arndb.de
Cc: linux-api@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: luto@kernel.org
Cc: akpm@linux-foundation.org
Cc: torvalds@linux-foundation.org
Link: http://lkml.kernel.org/r/20160729163023.407672D2@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>