MINOR: quic: restore QUIC_HP_SAMPLE_LEN constant

The below patch fixes padding emission for small packets, which is
required to ensure that header protection removal can be performed by
the recipient.

  commit d7dea408c6
  BUG/MINOR: quic: too short PADDING frame for too short packets

In addition to the proper fix, constant QUIC_HP_SAMPLE_LEN was removed
and replaced by QUIC_TLS_TAG_LEN. However, it still makes sense to have
a dedicated constant which represent the size of the sample used for
header protection. Thus, this patch restores it.

Special instructions for backport : above patch mentions that no
backport is needed. However, this is incorrect, as bug is introduced by
another patch scheduled for backport up to 2.6. Thus, it is first
mandatory to schedule d7dea408c6 after it.
Then, this patch can also be used for the sake of code clarity.

include/haproxy/quic_conn-t.h

@@ -145,6 +145,9 @@ enum quic_pkt_type {
 #define QUIC_PACKET_PNL_BITMASK      0x03
 #define QUIC_PACKET_PN_MAXLEN        4
 
+/* TLS algo supported by QUIC uses a 16-bytes sample for HP. */
+#define QUIC_HP_SAMPLE_LEN           16
+
 /*
  *  0                   1                   2                   3
  *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

src/quic_tx.c

@@ -1998,8 +1998,8 @@ static int qc_do_build_pkt(unsigned char *pos, const unsigned char *end,
 	 * Note that from here, <len> includes <*pn_len>, the total frame lenghts,
 	 * and QUIC_TLS_TAG_LEN(16).
 	 */
-	if (len < QUIC_PACKET_PN_MAXLEN + QUIC_TLS_TAG_LEN) {
-		padding_len = QUIC_PACKET_PN_MAXLEN + QUIC_TLS_TAG_LEN - len;
+	if (len < QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN) {
+		padding_len = QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN - len;
 		TRACE_PRINTF(TRACE_LEVEL_DEVELOPER, QUIC_EV_CONN_PHPKTS, qc, 0, 0, 0,
 		             "adding padding pn=%llu padding_len=%zu *pn_len=%zu"
 		             " len=%zu len_frms=%zu",