btrfs-progs: convert: Introduce function to calculate the available space

btrfs-progs: convert: Introduce function to calculate the available space

Introduce a new function, calculate_available_space() to get available
space cache_tree data_chunks cache_tree.

Unlike old implementation, this function will do the new work:

1) batch used ext* data space.
   To ensure data chunks will recovery them all.
   And restore the result into mkfs_cfg->convert_data_chunks for later
   use.

2) avoid SB and reserved space at chunk level
   Both batched data space or free space will not cover reserved space,
   like SB or the first 1M.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>