x86/mm: Switch from TASK_SIZE to TASK_SIZE_MAX in the page fault code

x86's page fault handlers had two TASK_SIZE uses that should have
been TASK_SIZE_MAX.  I don't think that either one had a visible
effect, but this makes the code clearer and should save a few bytes
of text.

(And I eventually want to eradicate TASK_SIZE.  This will help.)

Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ruslan Kabatsayev <b7.10110111@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1242fb23b0d05c3069dbf5758ac55d26bc114bef.1462914565.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>