update sources to Ubuntu-4.15.0-29.31

(generated with debian/scripts/import-upstream-tag)

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/submodules/ubuntu-bionic b/submodules/ubuntu-bionic
index 084f3ff49a0442309b617de066a1fe0223ace049..2fb721603a7e05f9ff2fde8f9335a76a91ba8fa2 160000 (submodule)
--- a/submodules/ubuntu-bionic
+++ b/submodules/ubuntu-bionic
@@ -1 +1 @@
-Subproject commit 084f3ff49a0442309b617de066a1fe0223ace049
+Subproject commit 2fb721603a7e05f9ff2fde8f9335a76a91ba8fa2