libcephfs/test.cc: close fd before umount

Fixes: #10415
Signed-off-by: Yan, Zheng <zyan@redhat.com>
(cherry picked from commit d3fb563cee4c4cf08ff4ee01782e52a100462429)
Merge pull request #3267 from ceph/emperor-10415

libcephfs/test.cc: close fd before umount