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>