block: fix alignment calculations in bdrv_co_do_zero_pwritev

block: fix alignment calculations in bdrv_co_do_zero_pwritev

tail_padding_bytes is calculated wrong. F.e. for
    offset = 0
    bytes = 2048
    align = 512
we will have tail_padding_bytes = 512 which is definitely wrong. The patch
fixes that arithmetics.

Fortunately this problem is harmless, we will have 1 extra allocation and
free thus there is no need to put this into stable. The problem is here
from the very beginning.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>