ipip: fix one sparse error

ipip: fix one sparse error

make C=2 CF=-D__CHECK_ENDIAN__ net/ipv4/ipip.o
  CHECK   net/ipv4/ipip.c
net/ipv4/ipip.c:254:27: warning: incorrect type in assignment (different base types)
net/ipv4/ipip.c:254:27:    expected restricted __be32 [addressable] [usertype] o_key
net/ipv4/ipip.c:254:27:    got restricted __be16 [addressable] [usertype] i_flags

Fixes: 3b7b514f44bf ("ipip: fix a regression in ioctl")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>