ci: add script to fetch kernel trees for libbpf sync

Set up a Linux repo the way sync-kernel.sh expects it: HEAD at
bpf-next's master and a bpf-master branch at bpf's master.

Fetch full history of both trees from the kernel-patches/bpf mirror on
GitHub, which is more reliable than git.kernel.org. The mirror may lag
behind, so don't require its branches to match git.kernel.org exactly,
only check that they're in the history of the master branches there.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>

.github/scripts/sync-fetch-kernel.sh

@@ -0,0 +1,75 @@
+#!/bin/bash
+#
+# Set up a Linux repo the way scripts/sync-kernel.sh expects it (see SYNC.md):
+# HEAD at bpf-next's master, and a local bpf-master branch at bpf's master.
+#
+# Both trees are fetched from the kernel-patches/bpf mirror on GitHub, which
+# is more reliable than git.kernel.org. The mirror may lag behind, but its
+# branches have to be in the history of master branches on git.kernel.org.
+
+usage () {
+	echo "USAGE: $0 <libbpf-repo> <linux-repo>"
+	echo ""
+	echo "<linux-repo> is created and must not exist yet."
+	echo "Set MIRROR_URL, BPF_NEXT_URL and BPF_URL to fetch from and check against other repos."
+	exit 1
+}
+
+set -euo pipefail
+
+LIBBPF_REPO=${1-""}
+LINUX_REPO=${2-""}
+
+if [ -z "${LIBBPF_REPO}" ] || [ -z "${LINUX_REPO}" ]; then
+	usage
+fi
+
+# The mirror's bpf-next and bpf branches track master of the respective trees
+MIRROR_URL=${MIRROR_URL:-https://github.com/kernel-patches/bpf.git}
+BPF_NEXT_URL=${BPF_NEXT_URL:-https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git}
+BPF_URL=${BPF_URL:-https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git}
+
+die()
+{
+	echo "Error: $*" >&2
+	exit 1
+}
+
+# Fetching just the commit by its ID wouldn't do: it's already here, and
+# git.kernel.org repos share objects, so it could come from another tree.
+# $1 - mirror's branch
+# $2 - git.kernel.org repo URL
+check_upstream()
+{
+	git fetch -q --no-tags "$2" refs/heads/master || die "can't fetch master from $2"
+	git merge-base --is-ancestor "$1" FETCH_HEAD ||
+		die "mirror's $1 ($(git rev-parse --short "$1")) is not in the history of master in $2"
+}
+
+[ ! -e "${LINUX_REPO}" ] || die "${LINUX_REPO} already exists"
+BPF_NEXT_BASE=$(cat "${LIBBPF_REPO}/CHECKPOINT-COMMIT")
+BPF_BASE=$(cat "${LIBBPF_REPO}/BPF-CHECKPOINT-COMMIT")
+
+git init -q -b master "${LINUX_REPO}"
+cd "${LINUX_REPO}"
+git config gc.auto 0
+
+echo "Fetching bpf-next and bpf from ${MIRROR_URL}..."
+git fetch -q --no-tags "${MIRROR_URL}" \
+	+refs/heads/bpf-next:refs/remotes/bpf-next/master \
+	+refs/heads/bpf:refs/remotes/bpf/master
+
+echo "Checking them against git.kernel.org..."
+check_upstream bpf-next/master "${BPF_NEXT_URL}"
+check_upstream bpf/master "${BPF_URL}"
+
+git merge-base --is-ancestor "${BPF_NEXT_BASE}" bpf-next/master ||
+	die "checkpoint ${BPF_NEXT_BASE} is not in bpf-next's history; if it was rebased away upstream, update the checkpoint manually"
+git merge-base --is-ancestor "${BPF_BASE}" bpf/master ||
+	die "checkpoint ${BPF_BASE} is not in bpf's history; if it was rebased away upstream, update the checkpoint manually"
+
+git checkout -q -B master bpf-next/master
+git branch -q -f bpf-master bpf/master
+
+echo "bpf-next tip: $(git log -n1 --pretty='%h ("%s")' master)"
+echo "bpf tip:      $(git log -n1 --pretty='%h ("%s")' bpf-master)"