powerpc/pseries: use kfree_sensitive() in plpks_gen_password()

password might contain private information, so better use
kfree_sensitive to free it.
In plpks_gen_password() use kfree_sensitive().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230717092648.9752-1-duminjie@vivo.com