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policy: Fix up selectorcache locking issue

When adding a new FQDN selector to the selector cache, the selector
cache lock would be held while registering into the DNS subsystem for
identity updates. This could then trigger allocation of CIDR identities,
leading to a call to UpdateIdentities() to notify all other selectors in
the cache about the new identity. This would then attempt to lock the
selector cache lock which was held at the beginning of this process,
leading to deadlock.

Fix the issue by releasing the selectorcache lock for the duration of
registering (and potentially triggering allocations), then after that
process is done, lock again and see if another goroutine has
successfully created the FQDN. If so, we can unregister from the proxy
again and use the existing selector; otherwise we can go ahead and
configure the selector in the selector cache and return it.

Thanks to Ian Vernon and Thomas Graf for their assistance digging to the
bottom of this issue.

Fixes: #9292

Signed-off-by: Joe Stringer <joe@cilium.io>

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