Unmap the mapped address region

It does not unmap the mapped address region,so we sometimes still get
such
warning when cleanup with nfs as TMPDIR although the file descriptor is
closed:
0  TWARN  :  tst_rmdir: rmobj(/mnt/nfsv3/ltp-MOeIFH7102/vma4FwRqb)
failed:
unlink(/mnt/nfsv3/ltp-MOeIFH7102/vma4FwRqb/.nfs000000000110cb3600000001)
failed; errno=16: Device or resource busy

Signed-off-by: Shuang Qiu <shuang.qiu@oracle.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>