x86/tdx: Fix data leak in mmio_read()

The mmio_read() function makes a TDVMCALL to retrieve MMIO data for an
address from the VMM.

Sean noticed that mmio_read() unintentionally exposes the value of an
initialized variable (val) on the stack to the VMM.

This variable is only needed as an output value. It did not need to be
passed to the VMM in the first place.

Do not send the original value of *val to the VMM.

[ dhansen: clarify what 'val' is used for. ]

Fixes: 31d58c4e ("x86/tdx: Handle in-kernel MMIO")
Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240826125304.1566719-1-kirill.shutemov%40linux.intel.com