filesync: reuse data buffer for diffcopy #5218

Job	Run time
govulncheck	1m 2s
prepare	5s
binaries (darwin/amd64)	1m 5s
binaries (darwin/arm64)	1m 2s
binaries (linux/amd64)	2m 1s
binaries (linux/arm/v7)	2m 1s
binaries (linux/arm64)	2m 3s
binaries (linux/s390x)	2m 1s
binaries (linux/ppc64le)	1m 58s
binaries (linux/riscv64)	2m 3s
binaries (windows/amd64)	1m 50s
binaries (windows/arm64)	1m 51s
test / prepare	1m 51s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 38s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	9m 12s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 43s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	9m 10s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 2s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 39s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 32s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 12s
test / run (./..., 1, integration gateway)	4m 21s
test / run (./client, containerd, nydus, integration)	7m 34s
test / run (./client, oci, nydus, integration)	4m 53s
test / run (./..., nydus, 1, integration)	4m 11s
image	9m 19s
image (rootless)	9m 12s
release	6s
scout	0s
	1h 55m 37s

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