bpf: prevent leak of lsm program after failed attach

In [0], we added the ability to bpf_prog_attach LSM programs to cgroups,
but in our validation to make sure the prog is meant to be attached to
BPF_LSM_CGROUP, we return too early if the check fails. This results in
lack of decrementing prog's refcnt (through bpf_prog_put)
leaving the LSM program alive past the point of the expected lifecycle.
This fix allows for the decrement to take place.

[0] https://lore.kernel.org/all/20220628174314.1216643-4-sdf@google.com/

Fixes: 69fd337a ("bpf: per-cgroup lsm flavor")
Signed-off-by: Milan Landaverde <milan@mdaverde.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221213175714.31963-1-milan@mdaverde.com

kernel/bpf/syscall.c

@@ -3518,8 +3518,8 @@ static int bpf_prog_attach(const union bpf_attr *attr)
 	case BPF_PROG_TYPE_LSM:
 		if (ptype == BPF_PROG_TYPE_LSM &&
 		    prog->expected_attach_type != BPF_LSM_CGROUP)
-			return -EINVAL;
-
+			ret = -EINVAL;
+		else
 			ret = cgroup_bpf_prog_attach(attr, ptype, prog);
 		break;
 	default: