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[9.20] fix: dev: Fix a data race between dns_zone_getxfr() and dns_xfrin_create()

bind-team-rw-api-rw-repo requested to merge backport-5011-dns_zone_getxfr-race-fix-9.20 into bind-9.20

There is a data race between the statistics channel, which uses dns_zone_getxfr() to get a reference to zone->xfr, and the creation of zone->xfr, because the latter happens outside of a zone lock.

Split the dns_xfrin_create() function into two parts to separate the zone transfer starting part from the zone transfer object creation part. This allows us to attach the new object to a local variable first, then attach it to zone->xfr under a lock, and only then start the transfer.

Closes #5011 (closed)

Backport of MR !9716 (merged)

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