ext4: ext4_ext_convert_to_initialized bug found in extended FSX testing

This patch addresses bugs found while testing punch hole 
with the fsx test.  The patch corrects the number of blocks
that are zeroed out while splitting an extent, and also corrects
the return value to return the number of blocks split out, instead
of the number of blocks zeroed out.

This patch has been tested in addition to the following patches: 
[Ext4 punch hole v7]
[XFS Tests Punch Hole 1/1 v2] Add Punch Hole Testing to FSX

The test ran successfully for 24 hours.

Signed-off-by: Allison Henderson <achender@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>