ipv6: ipv6_select_ident() returns a __be32

ipv6_select_ident() returns a 32bit value in network order.

Fixes: 286c2349

 ("ipv6: Clean up ipv6_select_ident() and ip6_fragment()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>