[DCCP]: Introduces follows48 function

This adds a new function to see if two sequence numbers follow each
other.

Signed off by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dccp/dccp.h

@@ -81,6 +81,14 @@ static inline u64 max48(const u64 seq1, const u64 seq2)
 	return after48(seq1, seq2) ? seq1 : seq2;
 }
 
+/* is seq1 next seqno after seq2 */
+static inline int follows48(const u64 seq1, const u64 seq2)
+{
+	int diff = (seq1 & 0xFFFF) - (seq2 & 0xFFFF);
+
+	return diff==1;
+}
+
 enum {
 	DCCP_MIB_NUM = 0,
 	DCCP_MIB_ACTIVEOPENS,			/* ActiveOpens */