tunneling: Add checks for header cache lock.

tunneling: Add checks for header cache lock.

When updating the tunnel header cache, we need to hold a lock to
protect against concurrent access.  This adds annotations to
make sparse happy when we access the data without rcu_read_lock
and enables lockdep to verify that we have the correct lock.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>