lttng-modules: Fix build with older kernels

* forward port this patch from daisy:
  commit c11b29ff4f24af0445c3c6a694b8dc2037dcd7e4
  Author: Tom Zanussi <tom.zanussi@intel.com>
  Date:   Thu Mar 6 22:26:20 2014 -0600

    lttng-modules: Fix 3.14 bio tracepoints

* how is it possible that it's in daisy but not in master?

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>