block/graph-lock: Disable locking for now

Kevin Wolfrequested to merge
kmwolf/centos-qemu-kvm:bz2186725 into c9s

Merge Request Required Information

Summary of Changes

tl;dr is that graph locking introduced deadlocks in 8.0, and disabling it for now fixes them again. See patch 1 for the details.

Patch 2 is a prerequisite for the test case. Instead of reproducing the deadlock problem (which it unfortunately doesn't do reliably anyway, the timing seems hard to get right), I got NBD server crashes without it.

Approved Development Ticket

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2186725

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