BUG/MINOR: mux_quic: do not crash on SE_SHR_DRAIN shut

BUG/MINOR: mux_quic: do not crash on SE_SHR_DRAIN shut

qcm_strm_shut() callback was extended to ensure stream is properly
closed. Only SE_SHR_DRAIN mode was left unsupported as it was considered
not possible to encounter it. As such a BUG_ON() was added in shut
callback.

Since QUIC backend support, SE_SHR_DRAIN may be used on shut for checks.
As such, it is required to properly handle it. This patch removes the
BUG_ON() and treat SE_SHR_DRAIN similarly to SE_SHR_RESET.

This must be backported up to 3.3.