coda: Avoid partial allocation of sig_inputArgs

GCC does not like having a partially allocated object, since it cannot
reason about it for bounds checking when it is passed to other code.
Instead, fully allocate sig_inputArgs. (Alternatively, sig_inputArgs
should be defined as a struct coda_in_hdr, if it is actually not using
any other part of the union.) Seen under GCC 13:

../fs/coda/upcall.c: In function 'coda_upcall':
../fs/coda/upcall.c:801:22: warning: array subscript 'union inputArgs[0]' is partly outside array bounds of 'unsigned char[20]' [-Warray-bounds=]
  801 |         sig_inputArgs->ih.opcode = CODA_SIGNAL;
      |                      ^~

Cc: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: coda@cs.cmu.edu
Cc: codalist@coda.cs.cmu.edu
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230127223921.never.882-kees@kernel.org

fs/coda/upcall.c

@@ -791,7 +791,7 @@ static int coda_upcall(struct venus_comm *vcp,
 	sig_req = kmalloc(sizeof(struct upc_req), GFP_KERNEL);
 	if (!sig_req) goto exit;
 
-	sig_inputArgs = kvzalloc(sizeof(struct coda_in_hdr), GFP_KERNEL);
+	sig_inputArgs = kvzalloc(sizeof(*sig_inputArgs), GFP_KERNEL);
 	if (!sig_inputArgs) {
 		kfree(sig_req);
 		goto exit;