x86/pkeys/selftests: Rename 'si_pkey' to 'siginfo_pkey'

'si_pkey' is now #defined to be the name of the new siginfo field that
protection keys uses.  Rename it not to conflict.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171111001231.DFFC8285@viggo.jf.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>