Btrfs-progs: add feature to get mininum size for resizing a fs/device

Currently there is not way for a user to know what is the minimum size a
device of a btrfs filesystem can be resized to. Sometimes the value of
total allocated space (sum of all allocated chunks/device extents), which
can be parsed from 'btrfs filesystem show' and 'btrfs filesystem usage',
works as the minimum size, but sometimes it does not, namely when device
extents have to relocated to holes (unallocated space) within the new
size of the device (the total allocated space sum).

This change adds the ability to reliably compute such minimum value and
extents 'btrfs filesystem resize' with the following syntax to get such
value:

   btrfs filesystem resize [devid:]get_min_size

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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