btrfs-progs: Cleanup old btrfs-convert

btrfs-progs: Cleanup old btrfs-convert

Cleanup all the old btrfs-convert facilities, including:
1) btrfs_convert_operations->alloc/free/test_extents*
   No need to do non-standard extent allocation.
   After init_btrfs() everything can be done by normal routine.

   Now only 4 functions are needed in btrfs_convert_operations.
   1) open_fs
   2) read_used_space
   3) copy_inodes
   4) close_fs

2) fs_info->extent_ops
   Same as above.

3) Old init_btrfs(), create_image(), create_file_image_range()
   Replaced with newer and cleaner one.

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