f2fs: support synchronous gc in ioctl

This patch drops in batches gc triggered through ioctl, since user
can easily control the gc by designing the loop around the ->ioctl.

We support synchronous gc by forcing using FG_GC in f2fs_gc, so with
it, user can make sure that in this round all blocks gced were
persistent in the device until ioctl returned.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -295,7 +295,7 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi)
 	 */
 	if (has_not_enough_free_secs(sbi, 0)) {
 		mutex_lock(&sbi->gc_mutex);
-		f2fs_gc(sbi);
+		f2fs_gc(sbi, false);
 	}
 }