logging: fix log buffer pending while it is definitely full

[xiaolusu]

The buffer flags MPSC_PBUF_FULL is a shared resource to determine whether the buffer is available. When processing drop messages, the flag will be cleared first and then set, add spin lock protection.

[github-actions]

Hello @xiaolusu, and thank you very much for your first pull request to the Zephyr project!
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[xiaolusu]

I think it can solve the problem mentioned here: #75736

[xiaolusu]

Hi, is there some action needed from my side for this pull request? why did the workflow pending?

[github-actions]

Hi @xiaolusu!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁