locking: Reorder functions in spinlock.c

Separate spin_lock and rw_lock functions. Preempt-RT needs to exclude
the rw_lock functions from being compiled. The reordering allows to do
that with a single #ifdef.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Ingo Molnar <mingo@elte.hu>