l2tp: remove incorrect __rcu attribute

This fixes a sparse warning.

Fixes: d18d3f0a

 ("l2tp: replace hlist with simple list for per-tunnel session list")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406220754.evK8Hrjw-lkp@intel.com/
Signed-off-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/20240624082945.1925009-1-jchapman@katalix.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>