ext4: Convert ext4_extent.ee_start to ext4_extent.ee_start_lo

Convert ext4_extent.ee_start to ext4_extent.ee_start_lo
This helps in finding BUGs due to direct partial access of
these split 48 bit values

Also fix direct partial access in ext4 code

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/include/linux/ext4_fs_extents.h b/include/linux/ext4_fs_extents.h
index 81406f3..cb2dfbbc 100644
--- a/include/linux/ext4_fs_extents.h
+++ b/include/linux/ext4_fs_extents.h
@@ -74,7 +74,7 @@
 	__le32	ee_block;	/* first logical block extent covers */
 	__le16	ee_len;		/* number of blocks covered by extent */
 	__le16	ee_start_hi;	/* high 16 bits of physical block */
-	__le32	ee_start;	/* low 32 bits of physical block */
+	__le32	ee_start_lo;	/* low 32 bits of physical block */
 };
 
 /*