powerpc: Use unlocked ioctl in nvram_64

The ioctl is only used for powermac systems and reads a partition
number from an array which is initialized at boot time way before the
nvram code is initialized. So it's safe to switch to unlocked_ioctl.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>