x86/speculation: Fix up array_index_nospec_mask() asm constraint

x86/speculation: Fix up array_index_nospec_mask() asm constraint

BugLink: http://bugs.launchpad.net/bugs/1751131
commit be3233fbfcb8f5acb6e3bcd0895c3ef9e100d470 upstream.

Allow the compiler to handle @size as an immediate value or memory
directly rather than allocating a register.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/151797010204.1289.1510000292250184993.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>