common/populate: decrease the step of rm file

common/populate: decrease the step of rm file

 Now that we have allocated 2*4096*64/16(32768) inodes after "Inode btree",
 but the step of rm file is too large to create enough free inodes in agi.
 So the freecount is not enough large to make free_level gt 1 and call
 _scratch__populate on xfs will report the following failure(such as xfs/083):

Failed to create fino of sufficient height!

By decreasing the step of rm file, xfs/083 will pass.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>