test: fix misc fiemap testing

test: fix misc fiemap testing

1) Different filesystem will have different behavior to
allocate extents. Therefore, even if write 4000 extents,
the filesystem may not really allocate 4000 extents.

2) kstore always return [0, xxx] even if offset ~= 0. Therefore,
the whole non-zero offset FiemapHoles test should be skipped

3) enable fiemap test for filestore, bluestore, memstore again

Fixes: http://tracker.ceph.com/issues/21716
Signed-off-by: Ning Yao <yaoning@unitedstack.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>