ext4: Calculate required journal credits for inserting an extent properly

When we have space in the extent tree leaf node we should be able to
insert the extent with much less journal credits. The code was doing
proper calculation but missed a return statement.

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