btrfs resize improvement for multiple devices

[cmurf]

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A btrfs volume can be made of multiple devices. Resize is implicit in 'btrfs dev add/remove' in the simple case. But if the user wants to grow or shrink without changing the number of devices, it's necessary to specify which device to apply the resize operation to.

btrfs filesystem resize [:][+/-][kKmMgGtTpPeE]|[:]max The devid can be found in the output of btrfs filesystem show and defaults to 1 if not specified.