Adopt nvram module to reduce code duplication. This means CONFIG_NVRAM
becomes available to PPC64 builds. Previously it was only available to
PPC32 builds because it depended on CONFIG_GENERIC_NVRAM.

The IOC_NVRAM_GET_OFFSET ioctl as implemented on PPC64 validates the
offset returned by pmac_get_partition(). Do the same in the nvram module.

Note that the old PPC32 generic_nvram module lacked this test.
So when CONFIG_PPC32 && CONFIG_PPC_PMAC, the IOC_NVRAM_GET_OFFSET ioctl
would have returned 0 (always). But when CONFIG_PPC64 && CONFIG_PPC_PMAC,
the IOC_NVRAM_GET_OFFSET ioctl would have returned -1 (which is -EPERM)
when the requested partition was not found.

With this patch, the result is now -EINVAL on both PPC32 and PPC64 when
the requested PowerMac NVRAM partition is not found. This is a userspace-
visible change, in the non-existent partition case, which would be in
an error path for an IOC_NVRAM_GET_OFFSET ioctl syscall.
Tested-by: NStan Johnson <userm57@yahoo.com>
Signed-off-by: NFinn Thain <fthain@telegraphics.com.au>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>