sparc: Replace one-element array with flexible array member

Replace the deprecated one-element array with a modern flexible array
member in the struct hvtramp_descr.

Additionally, 15 unnecessary bytes were allocated for hdesc, but instead
of fixing the parentheses in the open-coded version, use struct_size()
to calculate the correct number of bytes.

Link: https://github.com/KSPP/linux/issues/79


Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Fixes: 64658743 ("[SPARC64]: Remove most limitations to kernel image size.")
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/20241111204724.165263-2-thorsten.blum@linux.dev


Signed-off-by: Andreas Larsson <andreas@gaisler.com>