ipv4: Make ip_idents_reserve static

ip_idents_reserve is only used in net/ipv4/route.c. Make it static
and remove the export.

Signed-off-by: David Ahern <dsahern@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>