dup2/dup205: close all fds before tst_rmdir()

Close all open fds before calling tst_rmdir() to avoid TWARN caused by
NFS silly rename when removing temp dir.

dup205      0  TWARN  :  tst_rmdir: rmobj(/mnt/nfs/dupbA5J5i) failed: unlink(/mnt/nfs/dupbA5J5i/.nfs0000000000f4805b00000005) failed; errno=16: Device or resource busy

Signed-off-by: Eryu Guan <eguan@redhat.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Caspar Zhang <caspar@casparzhang.com>