l2tp: zero out stats in pppol2tp_copy_stats()

Integrate memset(0) in pppol2tp_copy_stats() to avoid calling it
manually every time.

While there, constify 'stats'.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>