libblkid: use fstatat(), improve readdir() usage

libblkid: use fstatat(), improve readdir() usage

 * fix possible memory leak in ubi_probe_all()

 * use dirent->d_type if available (this is tricky, because d_type
   is not supported on all systems and some filesystems returns
   DT_UNKNOWN). The dirent->d_type allows to avoid unnecessary stat()
   calls.

 * use fstatat() if available -- allows to avoid malloc() and
   sprintf("%s/%s", dirname, dirent->d_name)

Signed-off-by: Karel Zak <kzak@redhat.com>