lttng-modules: update 2.13.7 -> 2.13.8

Drop backported 0001-fix-mm-slab_common-drop-kmem_alloc-avoid-dereferenci.patch
0009-Rename-genhd-wrapper-to-blkdev.patch is present only on the master branch
and not on 2.13 branch, so keep it in the recipe.

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>