fs/mount_setattr: always cleanup mount_kattr

In the Linux kernel, the following vulnerability has been resolved:

fs/mount_setattr: always cleanup mount_kattr

Make sure that finish_mount_kattr() is called after mount_kattr was

succesfully built in both the success and failure case to prevent

leaking any references we took when we built it. We returned early if

path lookup failed thereby risking to leak an additional reference we

took when building mount_kattr when an idmapped mount was requested.