[PATCH] Merge two rcu types

[Jason A. Donenfeld]

Ideally the way to fix this would be to replace our usages with the
proper non-_bh ones, and then in compat, based on a version
comparison, define the non-_bh to the _bh. The problem is that
ratelimiter.c uses non-_bh already, and it's unclear how bad it would
be if this suddenly became _bh on old kernels.

IIRC, the _bh variant of those functions has been aliased to non-_bh
since a few versions. Do you know the first time it was the same?
Perhaps if that's far enough back, then it'd be worth the effort to
try to qualify the breakage of improperly _bh-ing ratelimiter.c would
be on those kernels. On the other hand, if it's too recent or if the
breakage would actually be significant, we could always fallback to
the `#ifndef COMPAT_CANNOT_*` pattern used in a few other places
(which gets stripped out automatically for upstream submissions).