Making sure ee_block is initialized to zero to prevent gcc from
kvetching. It's harmless (although it's not obvious that it's
harmless) from code inspection:
fs/ext4/move_extent.c:478: warning: 'start_ext.ee_block' may be used
uninitialized in this function
Thanks to Stefan Richter for first bringing this to the attention of
linux-ext4@vger.kernel.org.