BUG/MEDIUM: stick-tables: Make sure not to free a pending entry

BUG/MEDIUM: stick-tables: Make sure not to free a pending entry

There is a race condition, an entry can be free'd by stksess_kill()
between the time stktable_add_pend_updates() gets the entry from the
mt_list, and the time it adds it to the ebtree.
To prevent this, use the newly implemented MT_LIST_POP_LOCKED() to keep
the stksess locked until it is added to the tree. That way,
__stksess_kill() will wait until we're done with it.

This should be backported to 3.2.