Destroy serial_minors IDR on module exit, reclaiming the allocated memory.

This was detected by the following semantic patch (written by Luis
Rodriguez <mcgrof@suse.com>)

<SmPL>
@ defines_module_init @
declarer name module_init, module_exit;
declarer name DEFINE_IDR;
identifier init;
@@

module_init(init);

@ defines_module_exit @
identifier exit;
@@

module_exit(exit);

@ declares_idr depends on defines_module_init && defines_module_exit @
identifier idr;
@@

DEFINE_IDR(idr);

@ on_exit_calls_destroy depends on declares_idr && defines_module_exit @
identifier declares_idr.idr, defines_module_exit.exit;
@@

exit(void)
{
 ...
 idr_destroy(&idr);
 ...
}

@ missing_module_idr_destroy depends on declares_idr && defines_module_exit && !on_exit_calls_destroy @
identifier declares_idr.idr, defines_module_exit.exit;
@@

exit(void)
{
 ...
 +idr_destroy(&idr);
}
</SmPL>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: stable <stable@vger.kernel.org>	# v3.11
Signed-off-by: Johan Hovold <johan@kernel.org>