ext2: introduce helper for xattr entry validation

mainline inclusion
from mainline-5.3-rc1
commit f4c3fb8c
category: bugfix
bugzilla: 174001
CVE: NA

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

Introduce helper function ext2_xattr_entry_valid()
for xattr entry validation and clean up the entry
check related code.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>

Conflicts:
	fs/ext2/xattr.c

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>