KVM: TDX: Use struct_size to simplify tdx_get_capabilities()

KVM: TDX: Use struct_size to simplify tdx_get_capabilities()

Use struct_size() instead of manually calculating the number of bytes to
allocate for 'caps', including the nested flexible array, and copy all of
'caps' to user space with a single copy_to_user() call (thanks to the full
size being provided by struct_size()).

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20251017213914.167301-1-thorsten.blum@linux.dev
[sean: separate from swap of get_user() vs. kzalloc() ordering]
Signed-off-by: Sean Christopherson <seanjc@google.com>