lttng-modules: backport writeback.h changes from 2.12.x to fix kernel 5.4.62+
Backporting the kernel version changes for writeback.h to fix
compilation against kernel stable versions 5.4.62+
This is a combination of upstream commits:
87b2aff [fix: writeback: Fix sync livelock due to b_dirty_time processing (v5.9)]
b74b25f [fix: version ranges for ext4_discard_preallocations and writeback_queue_io
Upstream-status: Backport
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
