ext2: Strengthen xattr block checks

mainline inclusion
from mainline-5.3-rc1
commit 6c71b489
category: bugfix
bugzilla: 174001
CVE: NA

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

Check every entry in xattr block for validity in ext2_xattr_set() to
detect on disk corruption early. Also since e_value_block field in xattr
entry is never != 0 in a valid filesystem, just remove checks for it
once we have established entries are valid.

Reviewed-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>