ceph: fix type promotion bug on 32bit systems

In this code "ret" is type long and "src_objlen" is unsigned int.  The
problem is that on 32bit systems, when we do the comparison signed longs
are type promoted to unsigned int.  So negative error codes from
do_splice_direct() are treated as success instead of failure.

Cc: stable@vger.kernel.org
Fixes: 1b0c3b9f

 ("ceph: re-org copy_file_range and fix some error paths")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>