f2fs: introduce sbi->gc_mode to determine the policy

This is to avoid sbi->gc_thread pointer access.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.h

@@ -36,8 +36,6 @@ struct f2fs_gc_kthread {
 	unsigned int no_gc_sleep_time;
 
 	/* for changing gc mode */
-	unsigned int gc_idle;
-	unsigned int gc_urgent;
 	unsigned int gc_wake;
 };