tcp: define macros for a couple reclaim thresholds

A following patch is going to implement a similar reclaim schema
for the MPTCP protocol, with different locking.

Let's define a couple of macros for the used thresholds, so
that the latter code will be more easily maintainable.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sock.h

@@ -1573,6 +1573,11 @@ static inline void sk_mem_charge(struct sock *sk, int size)
 	sk->sk_forward_alloc -= size;
 }
 
+/* the following macros control memory reclaiming in sk_mem_uncharge()
+ */
+#define SK_RECLAIM_THRESHOLD	(1 << 21)
+#define SK_RECLAIM_CHUNK	(1 << 20)
+
 static inline void sk_mem_uncharge(struct sock *sk, int size)
 {
 	int reclaimable;
@@ -1589,8 +1594,8 @@ static inline void sk_mem_uncharge(struct sock *sk, int size)
 	 * If we reach 2 MBytes, reclaim 1 MBytes right now, there is
 	 * no need to hold that much forward allocation anyway.
 	 */
-	if (unlikely(reclaimable >= 1 << 21))
-		__sk_mem_reclaim(sk, 1 << 20);
+	if (unlikely(reclaimable >= SK_RECLAIM_THRESHOLD))
+		__sk_mem_reclaim(sk, SK_RECLAIM_CHUNK);
 }
 
 static inline void sk_wmem_free_skb(struct sock *sk, struct sk_buff *skb)