ubi: Correct the number of PEBs after a volume resize failure

In the error handling path `out_acc` of `ubi_resize_volume()`,
when `pebs < 0`, it indicates that the volume table record failed to
update when the volume was shrunk. In this case, the number of `ubi->avail_pebs`
and `ubi->rsvd_pebs` should be restored to their previous values to prevent
the UBI layer from reporting an incorrect number of available PEBs.

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>