f2fs: Fix deadlock in shutdown ioctl

f2fs_ioc_shutdown() ioctl gets stuck in the below path
when issued with F2FS_GOING_DOWN_FULLSYNC option.

__switch_to+0x90/0xc4
percpu_down_write+0x8c/0xc0
freeze_super+0xec/0x1e4
freeze_bdev+0xc4/0xcc
f2fs_ioctl+0xc0c/0x1ce0
f2fs_compat_ioctl+0x98/0x1f0

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1857,9 +1857,11 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
 	if (get_user(in, (__u32 __user *)arg))
 		return -EFAULT;
 
-	ret = mnt_want_write_file(filp);
-	if (ret)
-		return ret;
+	if (in != F2FS_GOING_DOWN_FULLSYNC) {
+		ret = mnt_want_write_file(filp);
+		if (ret)
+			return ret;
+	}
 
 	switch (in) {
 	case F2FS_GOING_DOWN_FULLSYNC:
@@ -1900,7 +1902,8 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
 
 	f2fs_update_time(sbi, REQ_TIME);
 out:
-	mnt_drop_write_file(filp);
+	if (in != F2FS_GOING_DOWN_FULLSYNC)
+		mnt_drop_write_file(filp);
 	return ret;
 }