ext4: do not use extent after put_bh

mainline inclusion
from mainline-v5.10-rc2
commit d7dce9e0
category: bugfix
bugzilla: 45516
CVE: NA

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

ext4_ext_search_right() will read more extent blocks and call put_bh
after we get the information we need.  However, ret_ex will break this
and may cause use-after-free once pagecache has been freed.  Fix it by
copying the extent structure if needed.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/r/20201028055617.2569255-1-yangerkun@huawei.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>