btrfs: derive compression type from extent map during reads

Derive the compression type from extent map as opposed to the bio flags
passed. This makes it more precise and not reliant on function
parameters.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>