ARM: avoid saving and restoring registers unnecessarily

Avoid repeatedly saving and restoring registers around the calls to
trace_hardirqs_on() and context_tracking_user_exit().  With the
previous changes, we no longer need to preserve "lr" across these
calls, and if we re-load r0-r3 later, we can avoid preserving these
regsiters too.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>