vmcore: clean up by removing unnecessary variable

vmcore: clean up by removing unnecessary variable

The variable j has int type but it's compared with u64 type.

Also, the purpose of the variable j is exactly what the variable real_sz
is used for now.  Replace the variable j by the variable real_sz and
remove the variable j.

Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>