ipv6: Don't increase IPSTATS_MIB_FRAGFAILS twice in ip6_fragment()

ipv6: Don't increase IPSTATS_MIB_FRAGFAILS twice in ip6_fragment()

RFC 2465 defines ipv6IfStatsOutFragFails as:

	"The number of IPv6 datagrams that have been discarded
	 because they needed to be fragmented at this output
	 interface but could not be."

The existing implementation, instead, would increase the counter
twice in case we fail to allocate room for single fragments:
once for the fragment, once for the datagram.

This didn't look intentional though. In one of the two affected
affected failure paths, the double increase was simply a result
of a new 'goto fail' statement, introduced to avoid a skb leak.
The other path appears to be affected since at least 2.6.12-rc2.

Reported-by: Sabrina Dubroca <sdubroca@redhat.com>
Fixes: 1d325d217c7f ("ipv6: ip6_fragment: fix headroom tests and skb leak")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>