BindMounts: fix to properly unmount during cleanup

The fs_bind testcases creates the mount directories for testing.
When the testcase exits, those directories were not properly unmounted.
The reason of the issues are:

 1. Some directories were kept mounted, even after the cleanup() routine, as the unmount code is missing.
 2. Some "umount" command execution fails, as they are dependent on some other mounted directories.

Error Report
------------
rbind/test02    1  TPASS  :  rbind/test02: PASSED
/kvt.new/ltp/testscripts/test_fs_bind.sh    0  TWARN  :  rbind/test02 did not properly clean up "/tmp/ltp-XXXX9qdYqG/sandbox"

rbind/test38    1  TPASS  :  rbind/test38: PASSED
/kvt.new/ltp/testscripts/test_fs_bind.sh    0  TWARN  :  rbind/test38 did not properly clean up "/tmp/ltp-XXXX9qdYqG/sandbox"

This patch add/reorder the unmount code to fix the above TWARN messages.

Modified-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Siva  <siva.padhy@lge.com>
Tested-by: Jan Stancek <jstancek@redhat.com>