btrfs: Cleanup and simplify find_newest_super_backup

Backup roots are always written in a circular manner. By definition we
can only ever have 1 backup root whose generation equals to that of the
superblock. Hence, the 'if' in the for loop will trigger at most once.
This is sufficient to return the newest backup root.

Furthermore the newest_gen parameter is always set to the generation of
the superblock. This value can be obtained from the fs_info.

This patch removes the unnecessary code dealing with the wraparound
case and makes 'newest_gen' a local variable.

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