powerpc: Fix lockdep IRQ tracing bug

A small bogon sneaked into the ppc64 lockdep support.  A test is
branching slightly off causing a clobbered register value to
overwrite the irq state under some circumstances.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>