pptp: fix sparse pointer warning

callid_sock array is referenced via rcu_dereference and
sparse rcu checks complains about address space mismatch.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/pptp.c

@@ -47,7 +47,7 @@
 #define MAX_CALLID 65535
 
 static DECLARE_BITMAP(callid_bitmap, MAX_CALLID + 1);
-static struct pppox_sock **callid_sock;
+static struct pppox_sock __rcu **callid_sock;
 
 static DEFINE_SPINLOCK(chan_lock);