xfstests: generic/315, verify preallocated disk space is released per truncate

xfstests: generic/315, verify preallocated disk space is released per truncate

Introduce generic test 315 to verify if the disk space is
released after truncating a preallocated file back to the
old smaller size.  Before Linux-3.10, Btrfs/OCFS2 test
failed in this case.

The test file is fallocated with FALLOC_FL_KEEP_SIZE option.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>