f2fs: fix to enable ATGC correctly via gc_idle sysfs interface

It needs to assign sbi->gc_mode with GC_IDLE_AT rather than GC_AT when
user tries to enable ATGC via gc_idle sysfs interface, fix it.

Fixes: 093749e ("f2fs: support age threshold based garbage collection")
Cc: Zhipeng Tan <tanzhipeng@hust.edu.cn>
Signed-off-by: Jicheng Shao <shaojicheng@hust.edu.cn>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -481,7 +481,7 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		} else if (t == GC_IDLE_AT) {
 			if (!sbi->am.atgc_enabled)
 				return -EINVAL;
-			sbi->gc_mode = GC_AT;
+			sbi->gc_mode = GC_IDLE_AT;
 		} else {
 			sbi->gc_mode = GC_NORMAL;
 		}