f2fs: fix to avoid NULL pointer dereference in f2fs_issue_flush()

With below two cases, it will cause NULL pointer dereference when
accessing SM_I(sbi)->fcc_info in f2fs_issue_flush().

a) If kthread_run() fails in f2fs_create_flush_cmd_control(), it will
release SM_I(sbi)->fcc_info,

- mount -o noflush_merge /dev/vda /mnt/f2fs
- mount -o remount,flush_merge /dev/vda /mnt/f2fs  -- kthread_run() fails
- dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync

b) we will never allocate memory for SM_I(sbi)->fcc_info w/ below
testcase,

- mount -o ro /dev/vda /mnt/f2fs
- mount -o rw,remount /dev/vda /mnt/f2fs
- dd if=/dev/zero of=/mnt/f2fs/file bs=4k count=1 conv=fsync

In order to fix this issue, let change as below:
- fix error path handling in f2fs_create_flush_cmd_control().
- allocate SM_I(sbi)->fcc_info even if readonly is on.
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -663,8 +663,7 @@ int f2fs_create_flush_cmd_control(struct f2fs_sb_info *sbi)
 	if (IS_ERR(fcc->f2fs_issue_flush)) {
 		int err = PTR_ERR(fcc->f2fs_issue_flush);
 
-		kfree(fcc);
-		SM_I(sbi)->fcc_info = NULL;
+		fcc->f2fs_issue_flush = NULL;
 		return err;
 	}
 
@@ -5138,11 +5137,9 @@ int f2fs_build_segment_manager(struct f2fs_sb_info *sbi)
 
 	init_f2fs_rwsem(&sm_info->curseg_lock);
 
-	if (!f2fs_readonly(sbi->sb)) {
-		err = f2fs_create_flush_cmd_control(sbi);
-		if (err)
-			return err;
-	}
+	err = f2fs_create_flush_cmd_control(sbi);
+	if (err)
+		return err;
 
 	err = create_discard_cmd_control(sbi);
 	if (err)