selftests/bpf: Add config dependency on BLK_DEV_LOOP

The ima selftest restricts its scope to a test filesystem image
mounted on a loop device and prevents permanent ima policy changes for
the whole system.

Fixes: 34b82d3a ("bpf: Add a selftest for bpf_ima_inode_hash")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201203191437.666737-4-kpsingh@chromium.org