sctp: remove old and unused SCTP_MIN_PMTU

This value is not used anywhere in the code. In essence it is a
duplicate of SCTP_DEFAULT_MINSEGMENT, which is used by the stack.

SCTP_MIN_PMTU value makes more sense, but we should not change to it now
as it would risk breaking applications.

So this patch removes SCTP_MIN_PMTU and adjust the comment above it.
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/constants.h

@@ -254,11 +254,10 @@ enum { SCTP_ARBITRARY_COOKIE_ECHO_LEN = 200 };
 #define SCTP_TSN_MAP_SIZE 4096
 
 /* We will not record more than this many duplicate TSNs between two
- * SACKs.  The minimum PMTU is 576.  Remove all the headers and there
- * is enough room for 131 duplicate reports.  Round down to the
+ * SACKs.  The minimum PMTU is 512.  Remove all the headers and there
+ * is enough room for 117 duplicate reports.  Round down to the
  * nearest power of 2.
  */
-enum { SCTP_MIN_PMTU = 576 };
 enum { SCTP_MAX_DUP_TSNS = 16 };
 enum { SCTP_MAX_GABS = 16 };