netfilter: nf_conntrack_ipv6: fix inconsistent lock state in nf_ct_frag6_gather()

[   63.531438] =================================
[   63.531520] [ INFO: inconsistent lock state ]
[   63.531520] 2.6.26-rc4 #7
[   63.531520] ---------------------------------
[   63.531520] inconsistent {softirq-on-W} -> {in-softirq-W} usage.
[   63.531520] tcpsic6/3864 [HC0[0]:SC1[1]:HE1:SE0] takes:
[   63.531520]  (&q->lock#2){-+..}, at: [<c07175b0>] ipv6_frag_rcv+0xd0/0xbd0
[   63.531520] {softirq-on-W} state was registered at:
[   63.531520]   [<c0143bba>] __lock_acquire+0x3aa/0x1080
[   63.531520]   [<c0144906>] lock_acquire+0x76/0xa0
[   63.531520]   [<c07a8f0b>] _spin_lock+0x2b/0x40
[   63.531520]   [<c0727636>] nf_ct_frag6_gather+0x3f6/0x910
 ...

According to this and another similar lockdep report inet_fragment
locks are taken from nf_ct_frag6_gather() with softirqs enabled, but
these locks are mainly used in softirq context, so disabling BHs is
necessary.

Reported-and-tested-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>