vmtest: #121 follow-ups. Loop increase bpf-next git fetch depth

- The previously introduced git fetch depth of bpf-next tree is not sufficient
when bpf-next tree is far ahead from libbpf checkpoint commit, so increase the
depth up to 128 max. Since 128 may be an overkill for a general case, increase
exponentially in a loop until max is reached.

- Do not fetch bpf-next twice
- Remove setup_example.sh

travis-ci/vmtest/prepare_selftests.sh

@@ -1,11 +1,19 @@
 #!/bin/bash
 
 set -eux
-GIT_FETCH_DEPTH="${GIT_FETCH_DEPTH}" ${VMTEST_ROOT}/checkout_latest_kernel.sh $1
+
+REPO_PATH=$1
+
+${VMTEST_ROOT}/checkout_latest_kernel.sh ${REPO_PATH}
+cd ${REPO_PATH}
+
+if [[ "${KERNEL}" = 'LATEST' ]]; then
+	cp ${VMTEST_ROOT}/configs/latest.config .config
+	make -j $((4*$(nproc))) olddefconfig all
+fi
 
 # Fix runqslower build
 # TODO(hex@): remove after the patch is merged from bpf to bpf-next tree
-cd $1
 wget https://lore.kernel.org/bpf/908498f794661c44dca54da9e09dc0c382df6fcb.1580425879.git.hex@fb.com/t.mbox.gz
 gunzip t.mbox.gz
 git apply t.mbox