replace nf_nat_seqofs_lock with spin_lock of nf_conn

[rajesh <rajesh.modhwadiya@elitecore.com>]

Hi,
     My kernel version is 2.6.32.
     I have verify that same lock is also used in 3.4.6.
     there is global spin lock at net/ipv4/netfilter/nf_nat_helper.c
     static DEFINE_SPINLOCK(nf_nat_seqofs_lock);

     this is used in following functions:
     adjust_tcp_sequence();
     nf_nat_get_offset();

     This lock will reduce performance in case of multicore machines.
     There is a spin_lock in nf_conn structure itself, which is used by
     tcp_packet() at
     net/netfilter/nf_conntrack_proto_tcp.c
     and other places.

     So can we replace
     spin_lock_bh(&nf_nat_seqofs_lock);
     with
     spin_lock_bh(&ct->lock);

     regards
     rajesh