CLEANUP: quic: compute the STREAM frame end offset once in qc_stream_frm_is_acked()

CLEANUP: quic: compute the STREAM frame end offset once in qc_stream_frm_is_acked()

The end offset of a STREAM frame, frm->offset + frm->len, was recomputed
four times in qc_stream_frm_is_acked(): once in the BUG_ON() and three
times in the offset comparisons. Store it in a local frm_end variable, as
already done for the FIN bit flag.

No functional change, no need to backport.