x86/mm/fault: Use TASK_SIZE_MAX in is_prefetch()

x86/mm/fault: Use TASK_SIZE_MAX in is_prefetch()

This is slightly shorter and slightly faster.  It's also more
correct: the split between user and kernel addresses is
TASK_SIZE_MAX, regardless of ti->flags.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/09156b63bad90a327827003c9e53faa82ef4c56e.1426728647.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>