seq_file: disallow extremely large seq buffer allocations

stable inclusion
from linux-4.19.198
commit 6de9f0bf7cacc772a618699f9ed5c9f6fca58a1d
CVE: CVE-2021-33909

--------------------------------

commit 8cae8cd8 upstream.

There is no reasonable need for a buffer larger than this, and it avoids
int overflow pitfalls.

Fixes: 058504ed ("fs/seq_file: fallback to vmalloc allocation")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>