UBIFS: remove useless @ecc in struct ubifs_scan_leb

We set @ecc in ubifs_scan_leb only if leb_read returns EBADMSG and
do not use it any more. This patch removes this variable and adds
comments about EBADMSG handling.

Artem: re-phrase commentaries

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>