selftests/bpf: Limit URLS parsing logic to actual scope in vmtest

selftests/bpf: Limit URLS parsing logic to actual scope in vmtest

The URLS array is only valid in the download_rootfs function and does
not need to be parsed globally in advance. At the same time, the logic
of loading rootfs is refactored to prepare vmtest for supporting local
rootfs.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-5-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>