kcore: fix /proc/kcore's stat.st_size

In 9063c61f ("x86, 64-bit: Clean up user address masking") Linus
fixed the wrong size of /proc/kcore problem.

But its size still looks insane, since it never equals the size of
physical memory.

Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Tao Ma <tao.ma@oracle.com>
Cc: <mtk.manpages@gmail.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>