mm: swap_ratio: Only call trace_printk() in debug

 * Use DEBUG preprocessor macro to get rid of the message seen below on dmesg.

 **********************************************************
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **                                                      **
 ** trace_printk() being used. Allocating extra memory.  **
 **                                                      **
 ** This means that this is a DEBUG kernel and it is     **
 ** unsafe for produciton use.                           **
 **                                                      **
 ** If you see this message and you are not debugging    **
 ** the kernel, report this immediately to your vendor!  **
 **                                                      **
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **********************************************************

 The usage of trace_printk() causes remarkable CPU load and memory waste
 on production systems, as described here: https://lkml.org/lkml/2014/5/28/496

Change-Id: Idf89e2a1405edf2efa6526d18892eaa7e9a6221b