ci: Add a step to patch kernel with temporary fixes

Apply a custom set of patches against bpf-next kernel tree before
building vmlinux image.

.github/actions/vmtest/action.yml

@@ -33,6 +33,11 @@ runs:
         repo: 'https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git'
         rev: ${{ env.CHECKPOINT }}
         dest: '${{ github.workspace }}/.kernel'
+    - name: Patch kernel source
+      uses: libbpf/ci/patch-kernel@master
+      with:
+        patches-root: '${{ github.workspace }}/travis-ci/diffs'
+        repo-root: '.kernel'
     - name: Prepare to build BPF selftests
       shell: bash
       run: |