btrfs: defrag: use control structure in btrfs_defrag_file()

btrfs: defrag: use control structure in btrfs_defrag_file()

Use defrag control structure and replace function arguments. This brings
the following benefits:

- No more strange range->start update to indicate last scanned bytenr
  We have btrfs_defrag_ctrl::last_scanned (exclusive) for it directly.

- No more return value to indicate defragged sectors
  Now btrfs_defrag_file() will just return 0 if no error happened.
  And btrfs_defrag_ctrl::sectors_defragged will show that value.

- Fewer parameters to carry around
  Now most defrag_* functions only need to fetch their policy parameters
  from btrfs_defrag_ctrl directly.

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