x86: rename do_not_nx to disable_nx in mm/init_64.c

As a preparational step for unifying noexec handling on 32-bit and 64-bit,
rename the do_not_nx variable to disable_nx on 64-bit.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1236265497.31324.11.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>