[POWERPC] spufs: Fix gang destroy leaks

Previously, closing a SPE gang that still has contexts would trigger
a WARN_ON, and leak the allocated gang.

This change fixes the problem by using the gang's reference counts to
destroy the gang instead. The gangs will persist until their last
reference (be it context or open file handle) is gone.

Also, avoid using statements with side-effects in a WARN_ON().

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>