f2fs/gc: Reduce GC thread urgent sleep time to 50ms

Android sets the value to 50ms via vold's IdleMaint service. Since
500ms is too long for GC to colllect all invalid segments in time
which results in performance degradation.

On un-encrypted device, vold fails to set this value to 50ms thus
degrades the performance over time.

Change-Id: I80f2c29558393d726d5e696aaf285096c8108b23

fs/f2fs/gc.h

@@ -10,7 +10,7 @@
 						 * whether IO subsystem is idle
 						 * or not
 						 */
-#define DEF_GC_THREAD_URGENT_SLEEP_TIME	500	/* 500 ms */
+#define DEF_GC_THREAD_URGENT_SLEEP_TIME	50	/* 50 ms */
 #define DEF_GC_THREAD_MIN_SLEEP_TIME	30000	/* milliseconds */
 #define DEF_GC_THREAD_MAX_SLEEP_TIME	60000
 #define DEF_GC_THREAD_NOGC_SLEEP_TIME	300000	/* wait 5 min */