src/t_mtab: Add error check for unlock_mtab()

src/t_mtab: Add error check for unlock_mtab()

When unlink() fails, that is, when the lock file is not deleted
successfully, variable we_created_lockfile is still set to 0.

On the next iteration, the 3 processes will not be able to
successfully create the lock file.

Signed-off-by: Cui Yue <cuiyue-fnst@cn.fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>