net-tcp: Remove deadlocks and race conditions from TCP estats.

Ensure correct spin_locks are used throughout the TCP estats code, and
that returned pointers from idr_* are checked and locked before being
used. Add rcu_read_lock use and associated synchronization point.

Added some uses of jiffies_to_msec instead of manually converting.
Merge pull request #7 from rapier1/web10g.fix_locks

net-tcp: Remove deadlocks and race conditions from TCP estats.