coda: Avoid partial allocation of sig_inputArgs

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