open_by_handle: test directory file handle

open_by_handle: test directory file handle

usage: open_by_handle -cp <test_dir> [N]
usage: open_by_handle  -p <test_dir> [N]
usage: open_by_handle -dp <test_dir> [N]

With -p flag, create/delete also the test_dir itself and try to open by
handle also test_dir itself after droping caches and use dir fd to test
faccessat() of a file inside dir.
mount_fd argument to open_by_handle_at() is open fd of test_dir's parent.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>