l2tp: remove unneeded null check in l2tp_v2_session_get_next

Commit aa92c1cec92b ("l2tp: add tunnel/session get_next helpers") uses
idr_get_next APIs to iterate over l2tp session IDR lists. Sessions in
l2tp_v2_session_idr always have a non-null session->tunnel pointer
since l2tp_session_register sets it before inserting the session into
the IDR. Therefore the null check on session->tunnel in
l2tp_v2_session_get_next is redundant and can be removed. Removing the
check avoids a warning from lkp.

Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
CC: Dan Carpenter <[e-mail]>
Signed-off-by: James Chapman <[e-mail]>
Acked-by: Tom Parkin <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/l2tp/l2tp_core.c

···
 			goto again;
 		}
 
-		if (tunnel && tunnel->tunnel_id == tid &&
+		if (tunnel->tunnel_id == tid &&
 		    refcount_inc_not_zero(&session->ref_count)) {
 			rcu_read_unlock_bh();
 			return session;