x86: Use helpers for rlimits

Make sure compiler won't do weird things with limits.  Fetching them
twice may return 2 different values after writable limits are
implemented.

We can either use rlimit helpers added in
3e10e716 or ACCESS_ONCE if not
applicable; this patch uses the helpers.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
LKML-Reference: <1264609942-24621-1-git-send-email-jslaby@suse.cz>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>