f2fs: Fix uninitialized return in f2fs_ioc_shutdown()

"ret" can be uninitialized on the success path when "in ==
F2FS_GOING_DOWN_FULLSYNC".

Fixes: 60b2b4ee ("f2fs: Fix deadlock in shutdown ioctl")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>