IPAM: fix subnet mutex not released when static IP is out of range

[zhangzujian]

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What type of this PR

• Bug fixes

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at 8795e83

Fix concurrency issue in GetStaticAddress function. Use defer to unlock mutex and avoid potential deadlocks in pkg/ipam/subnet.go.

🤖 Generated by Copilot at 8795e83

There once was a function called GetStaticAddress
That had a concurrency issue to address
It used a mutex lock
But forgot to unlock
So it fixed it with a defer, no less

HOW

🤖 Generated by Copilot at 8795e83

• Add a deferred call to unlock the subnet mutex in GetStaticAddress (link)
• Remove the redundant explicit call to unlock the subnet mutex in GetStaticAddress (link)