btrfs: raid56: extract sector recovery code into a helper

This includes extra changes:

- The allocation for unmap_array[] and pointers[]
  Now we allocate them in one go, and free them together.

- Remove @err
  Use errno_to_blk_status(ret) instead.

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