mmapstress10:Close the file descriptor

It does not close the file descriptor before return 1 in fileokey()
function which may cause the following warning with nfs as TMPDIR:
0  TWARN  :  tst_rmdir: rmobj(/mnt/nfsv3/ltp-SDrfq17456/mmaq7TOqt)
failed:
unlink(/mnt/nfsv3/ltp-SDrfq17456/mmaq7TOqt/.nfs000000000110cb3600000004)
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>