DEBUG: channel: add consistency checks using BUG_ON_HOT() in some key functions

DEBUG: channel: add consistency checks using BUG_ON_HOT() in some key functions

A few functions such as c_adv(), c_rew(), co_set_data() or co_skip() got a
BUG_ON_HOT() to make sure they're not used to push more data than available
in the buffer. Note that with HTX the margin can be high and will less easily
trigger, but the goal is to detect a misuse early enough.

co_data() should never be called with a wrong c->output. At least it never
happens in regtests, but we're adding a CHECK_IF_HOT() there to avoid crashing
but report it if it ever happened when the hot path checks are enabled.