tests: sparse-fiemap: with root/ext3, do not create an ext4 FS

tests: sparse-fiemap: with root/ext3, do not create an ext4 FS

* tests/cp/sparse-fiemap: When this test was run as root on an ext3
file system, (ext3 had known problems), it would trickily create and
mount a loopback ext4 file system and use that instead.  However, due
to a bug in 2.6.39-rc1..rc3, this loopback test (when run in another
loopback FS) exposed a bug with 1k-blocksize ext4 whereby non-NUL
data would be read from a hole.  For details, see this:
http://thread.gmane.org/gmane.comp.file-systems.ext4/24495