powerpc/nvram: Fix a memory leak in err path

If kmemdup fails, We need kfree *buff* first then return -ENOMEM.
Otherwise there is a memory leak.

Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Reviewed-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>