LU-4416 tests: small fixes for conf_sanity

LU-4416 tests: small fixes for conf_sanity

--In util-linux 2.23.2, umount will call stat() on
  mountpoint. So have to use 'umount -f' in test_5a.
  Else the test will block forever.
--Invoke cleanup_fs2 in test_24b.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: Ie7540d6cfd6790417c63ea1efb50381cacc8345f
Reviewed-on: http://review.whamcloud.com/10783
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>