powerpc/nvram: Add missing kfree in error path

If 'nvram_write_header' fails, then 'new_part' should be freed, otherwise,
there is a memory leak.

Signed-off-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>