workspace: update linux to 6.12.15

Switch to the current 6.12 LTS branch and rebase the patches.
Also switch to using savedefconfig for config instead of just including
the entire thing. This makes it much more readable as it only contains
settings we've touched.

Change-Id: I2b15944f0083399290a2539c42e37205e839bf36
Reviewed-on: https://review.monogon.dev/c/monogon/+/3908
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/third_party/linux/patches/disable-static-ifs.patch b/third_party/linux/patches/disable-static-ifs.patch
index 8ffad1e..78ba491 100644
--- a/third_party/linux/patches/disable-static-ifs.patch
+++ b/third_party/linux/patches/disable-static-ifs.patch
@@ -1,3 +1,4 @@
+
 A few virtual drivers (bonding, dummy) predate netlink (~2003), which means that
 the kernel had no way to dynamically create network interfaces.
 The solution was kernel module paramter which statically precreated a list of