Btrfs: use 'u64' rather than 'int' to get extent's generation

We define a 'int' to get extent's generation by mistake,fix it.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>