loopback: Revert 04a0551c8 (Drop obsolete ip_summed setting)

Now that the network stack can handle inbound packets with partial
checksums, we should no longer clobber the ip_summed field in the
loopback driver.  This is because CHECKSUM_UNNECESSARY implies that
the checksum field is actually valid which is not true for loopback
packets since it's only partial (and thus complemented).

This allows packets from lo to then be SNATed to an external source
while still preserving the checksum's validity.

[Note from Zheng Liu]
Actually we can backport this commit (2e8e18ef52: tcp: Set
CHECKSUM_UNNECESSARY in tcp_init_nondata_skb) in order to solve the
problem that tfo doesn't work in loopback device.  But after appied
it the ack packet will be droped because of skb->ip_summed.  Thus,
we revert the commit.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>