ipv6: fix sparse errors in ip6_make_flowlabel()

include/net/ipv6.h:713:22: warning: incorrect type in assignment (different base types)
include/net/ipv6.h:713:22:    expected restricted __be32 [usertype] hash
include/net/ipv6.h:713:22:    got unsigned int
include/net/ipv6.h:719:25: warning: restricted __be32 degrades to integer
include/net/ipv6.h:719:22: warning: invalid assignment: ^=
include/net/ipv6.h:719:22:    left side has type restricted __be32
include/net/ipv6.h:719:22:    right side has type unsigned int

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>