l2tp(4): avoid having struct ifqueue directly in a percpu storage.
percpu(9) has a certain memory storage for each CPU and provides it by the piece
to users. If the storages went short, percpu(9) enlarges them by allocating new
larger memory areas, replacing old ones with them and destroying the old ones.
A percpu storage referenced by a pointer gotten via percpu_getref can be
destroyed by the mechanism after a running thread sleeps even if percpu_putref
has not been called.
Tx processing of l2tp(4) uses normally involves sleepable operations so we
must avoid dereferencing a percpu data (struct ifqueue) after executing Tx
processing. Address this situation by having just a pointer to the data in
a percpu storage instead.
Reviewed by ozaki-r@ and yamaguchi@
UnifiedSplitRaw
