x86/sev-es: Remove subtraction of res variable

vc_decode_insn() calls copy_from_kernel_nofault() by way of
vc_fetch_insn_kernel() to fetch 15 bytes max of opcodes to decode.

copy_from_kernel_nofault() returns negative on error and 0 on success.
The error case is handled by returning ES_EXCEPTION.

In the success case, the ret variable which contains the return value is
0 so there's no need to subtract it from MAX_INSN_SIZE when initializing
the insn buffer for further decoding. Remove it.

No functional changes.
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NJoerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/20210223111130.16201-1-bp@alien8.de

arch/x86/kernel/sev-es.c

@@ -267,7 +267,7 @@ static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
 			return ES_EXCEPTION;
 		}
 
-		insn_init(&ctxt->insn, buffer, MAX_INSN_SIZE - res, 1);
+		insn_init(&ctxt->insn, buffer, MAX_INSN_SIZE, 1);
 		insn_get_length(&ctxt->insn);
 	}