incusd/storage/btrfs: Fix btrfs argument order #1951

Job	Run time
Code	7m 23s
Documentation	6m 41s
Client (1.21.x, ubuntu-latest)	1m 4s
System (1.21.x, cluster, dir)	11m 47s
Client (1.21.x, macos-latest)	1m 6s
System (1.21.x, cluster, btrfs)	11m 53s
Client (1.21.x, windows-latest)	4m 20s
System (1.21.x, cluster, lvm)	12m 34s
System (1.21.x, cluster, zfs)	12m 24s
System (1.21.x, cluster, ceph)	17m 16s
System (1.21.x, cluster, random)	11m 44s
System (1.21.x, standalone, dir)	21m 17s
System (1.21.x, standalone, btrfs)	21m 22s
System (1.21.x, standalone, lvm)	40m 54s
System (1.21.x, standalone, zfs)	40m 5s
System (1.21.x, standalone, ceph)	1h 14m 11s
System (1.21.x, standalone, random)	21m 30s
System (stable, cluster, dir)	11m 31s
System (stable, standalone, dir)	20m 58s
System (tip, cluster, dir)	11m 54s
System (tip, standalone, dir)	21m 48s
	6h 23m 42s

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