NFSD: Use struct_size() helper in alloc_session()

Use struct_size() helper to simplify the code, no functional changes.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>