MINOR: quic: restore QUIC_HP_SAMPLE_LEN constant

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 d7dea408c64c327cab6aebf4ccad93405b675565
  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 d7dea408c64c327cab6aebf4ccad93405b675565 after it.
Then, this patch can also be used for the sake of code clarity.