IPoIB: Use spin_lock_irq() instead of spin_lock_irqsave()

We know ipoib_flush_paths() is called from plain process context with
interrupts enabled, since it does wait_for_completion().  So there's
no need to use spin_lock_irqsave() -- spin_lock_irq() is fine.

Signed-off-by: Roland Dreier <rolandd@cisco.com>