bpf: Avoid unnecessary audit log for CPU security mitigations

Check cpu_mitigations_off() first to avoid calling capable() if it is off.
This can avoid unnecessary audit log.

Fixes: bc5bc309 ("bpf: Inherit system settings for CPU security mitigations")
Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/CAEf4Bza6UVUWqcWQ-66weZ-nMDr+TFU3Mtq=dumZFD-pSqU7Ow@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20231013083916.4199-1-laoar.shao@gmail.com