block: Fix in_flight leak in request padding error path

Kevin Wolfrequested to merge
kmwolf/centos-qemu-kvm:bz1972079 into c9s

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1972079 Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=38724758

Fix a hang after forgetting bdrv_dec_in_flight() in a request padding error path (affects Windows + 512-on-4k emulation)

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