ringbuffer: fix mistaken errno handling around _rb_chunk_reclaim

Previously, there were two separate logical issues:

• errno could be set negative in qb_rb_chunk_alloc when when "reclaim" notifier failed

• _rb_chunk_reclaim (note: local scoped, hence comfortable for changes) was already setting errno at a single (coincidentally, in a correct way, but that'd be overwritten with the inverse because of the previous logical issue in qb_rb_chunk_alloc), so make it set errno at each failure path (now also when internal integrity in _rb_chunk_reclaim failed(), sparing the callers to double on that task

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>