configfs: add missing mutex_unlock()

d_alloc() failure in configfs_register_subsystem() would fail to unlock
the mutex taken above.  Reorganize the exit path to ensure the unlock
happens.

Reported-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>