MINOR: debug: unify __BUG_ON() and __BUG_ON_ONCE()

MINOR: debug: unify __BUG_ON() and __BUG_ON_ONCE()

We can do that thanks to the debug type. The line counter moved into an
if () block conditioned by the type so it continues to emit nothing for
regular bugs.

Surprizingly, the code grew by 1 kB. It's apparently caused by having
an impossible "if" statement containing a "break". Splitting the macro
in two to put the BUG_ON_ONCE() callers aside showed that having only
"if (0) break;" in the block was sufficient to make gcc deoptimize it
and result in this same growth. This was observed with 9.5. Let's
consider this a temporary limitation and keep it this way.