MEDIUM: mt_list: Reduce the max number of loops with exponential backoff

Reduce the max number of loops in the mt_list code while waiting for
a lock to be available with exponential backoff. It's been observed that
the current value led to severe performances degradation at least on
some hardware, hopefully this value will be acceptable everywhere.

include/import/mt_list.h

@@ -225,7 +225,7 @@ static inline __attribute__((always_inline)) unsigned long mt_list_cpu_relax(uns
 	/* limit maximum wait time for unlucky threads */
 	loop = mt_list_wait(loop);
 
-	for (loop &= 0x7fffff; loop >= 32; loop--) {
+	for (loop &= 0xfffff; loop >= 32; loop--) {
 #if defined(__x86_64__)
 		/* This is a PAUSE instruction on x86_64 */
 		asm volatile("rep;nop\n");