btrfs: use btrfs_path auto free in zoned.c

All cleanup paths lead to btrfs_path_free so path can be defined with
the automatic freeing callback in the following functions:

- calculate_emulated_zone_size()
- calculate_alloc_pointer()

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>