btrfs-progs: convert: Introduce function to migrate reserved ranges

btrfs-progs: convert: Introduce function to migrate reserved ranges

Introduce new function, migrate_reserved_ranges() to migrate used fs
data in btrfs reserved space.

Unlike old implementation, which will need to relocate all the
complicated csum and reference relocation, previous patches already
ensure such reserved ranges won't be allocated.  So here we only need
copy these data out and create new extent/csum/reference.

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