btrfs: print-tree: move code for processing file extent item into helper

The code for processing file extent items is quite large and it's better
to have it in a dedicated helper rather than in a huge switch statement,
just like we do in btrfs-progs.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>