proc: use WARN() rather than printk+backtrace

Use WARN() rather than a printk() + backtrace();
this gives a more standard format message as well as complete
information (including line numbers etc) that will be collected
by kerneloops.org

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>