swiotlb: do not zero buffer in set_memory_decrypted()

For larger TDX VM, memset() after set_memory_decrypted() in
swiotlb_update_mem_attributes() takes substantial portion of boot time.

Zeroing doesn't serve any functional purpose. Malicious VMM can mess
with decrypted/shared buffer at any point.

Remove the memset().
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/swiotlb.c

@@ -207,8 +207,6 @@ void __init swiotlb_update_mem_attributes(void)
 	mem->vaddr = swiotlb_mem_remap(mem, bytes);
 	if (!mem->vaddr)
 		mem->vaddr = vaddr;
-
-	memset(mem->vaddr, 0, bytes);
 }
 
 static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,