selftests/bpf: Fix cap_enable_effective() return code

selftests/bpf: Fix cap_enable_effective() return code

The caller of cap_enable_effective() expects negative error code.
Fix it.

Before:
  failed to restore CAP_SYS_ADMIN: -1, Unknown error -1

After:
  failed to restore CAP_SYS_ADMIN: -3, No such process
  failed to restore CAP_SYS_ADMIN: -22, Invalid argument

Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250305022234.44932-1-yangfeng59949@163.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>