btrfs-progs: utils: Introduce new function for convert

btrfs-progs: utils: Introduce new function for convert

Introduce new function make_convert_btrfs() for convert.

This new function will have the following features:
1) Allocate temporary sb/metadata/system chunk, avoiding old used data
2) More structured functions
   No more over 1000 lines function, better function split and code
   reuse

This will finally replace current make_btrfs(), but now only used for
convert.

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