tunnel: hh_cache access cleanup

tunnel: hh_cache access cleanup

Following patch cleanup hh_cache access by avoiding hh pointer fetching
most of time. Now hh is read and checked at beginning of function. All
hh->hh_len access are done inside hh_lock.
This is required cleanup for next patch which adds support for kernel
3.1.

Suggested-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Pravin Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>