ext4: Initialize writeback_index to 0 when allocating a new inode

The write_cache_pages() function uses the mapping->writeback_index as
the starting index to write out when range_cyclic is set.  Properly
initialize writeback_index so that we start the writeout at index 0.

This was found when debugging the small file fragmentation on ext4.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>