ipv6: fix a sparse warning

net/ipv6/reassembly.c:82:72: warning: incorrect type in argument 3 (different base types)
net/ipv6/reassembly.c:82:72:    expected unsigned int [unsigned] [usertype] c
net/ipv6/reassembly.c:82:72:    got restricted __be32 [usertype] id

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>