BpfLoader: split bpffs subdir creation out am: 7ff8310ea2

Original change: https://android-review.googlesource.com/c/platform/system/bpf/+/3217182

Change-Id: Ifc1fa2de2a4f2f542eeea3d0ac9a9bb0c4c18bc8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>