xfrm: Use kmem_cache_zalloc() instead of kmem_cache_alloc() with flag GFP_ZERO.

Use kmem_cache_zalloc instead of manually setting kmem_cache_alloc
with flag GFP_ZERO since kzalloc sets allocated memory
to zero.

Change in v2:
     add indation

Signed-off-by: Huang Zijiang <huang.zijiang@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>