xfrm: use correct size to initialise sp->ovec

This place should want to initialize array, not a element,
so it should be sizeof(array) instead of sizeof(element)

but now this array only has one element, so no error in
this condition that XFRM_MAX_OFFLOAD_DEPTH is 1

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>