This adds a basic test for bpf_override_return to verify it works. We
override the main function for mounting a btrfs fs so it'll return
-ENOMEM and then make sure that trying to mount a btrfs fs will fail.
Acked-by: Alexei Starovoitov
Acked-by: Ingo Molnar
Signed-off-by: Josef Bacik
Signed-off-by: Alexei Starovoitov