f2fs: Fix uninitialized return in f2fs_ioc_shutdown()

[ Upstream commit 2a96d8ad94ce57cb0072f7a660b1039720c47716 ]

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

Fixes: 60b2b4ee2bc0 ("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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>