ipv6: use READ_ONCE() for inet->hdrincl as in ipv4

As it was done in commit 8f659a03 ("net: ipv4: fix for a race
condition in raw_sendmsg") and commit 20b50d79

 ("net: ipv4: emulate
READ_ONCE() on ->hdrincl bit-field in raw_sendmsg()") for ipv4, copy the
value of inet->hdrincl in a local variable, to avoid introducing a race
condition in the next commit.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>