configfs: Prevent duplicate subsystem names.

For all child objects, creation comes through mkdir(2), so duplicate names
are prevented.

Subsystems, though, are registered by client drivers at init_module()/__init
time.  This patch prevents duplicate subsystem names.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>