mm/memfd_luo: remove unnecessary memset in zero-size memfd path

The memset(kho_vmalloc, 0, sizeof(*kho_vmalloc)) call in the zero-size
file handling path is unnecessary because the allocation of the ser
structure already uses the __GFP_ZERO flag, ensuring the memory is already
zero-initialized.

Link: https://lore.kernel.org/20260326084727.118437-4-duanchenghao@kylinos.cn
Signed-off-by: Chenghao Duan <duanchenghao@kylinos.cn>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Haoran Jiang <jianghaoran@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>