firewire: core: use struct_size and flex_array_size in ioctl_add_descriptor

firewire: core: use struct_size and flex_array_size in ioctl_add_descriptor

Use struct_size() to determine the memory needed for a new 'struct
descriptor_resource' and flex_array_size() to calculate the number of
bytes to copy from userspace. This removes the hardcoded size (4 bytes)
for the 'u32 data[]' entries.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250916122143.2459993-3-thorsten.blum@linux.dev
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>