wifi: iwlegacy: Fix "field-spanning write" warning in il_enqueue_hcmd()

iwlegacy uses command buffers with a payload size of 320
bytes (default) or 4092 bytes (huge).  The struct il_device_cmd type
describes the default buffers and there is no separate type describing
the huge buffers.

The il_enqueue_hcmd() function works with both default and huge
buffers, and has a memcpy() to the buffer payload.  The size of
this copy may exceed 320 bytes when using a huge buffer, which
now results in a run-time warning:

    memcpy: detected field-spanning write (size 1014) of single field "&out_cmd->cmd.payload" at drivers/net/wireless/intel/iwlegacy/common.c:3170 (size 320)

To fix this:

- Define a new struct type for huge buffers, with a correctly sized
  payload field
- When using a huge buffer in il_enqueue_hcmd(), cast the command
  buffer pointer to that type when looking up the payload field

Reported-by: Martin-Éric Racine <martin-eric.racine@iki.fi>
References: https://bugs.debian.org/1062421
References: https://bugzilla.kernel.org/show_bug.cgi?id=219124


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 54d9469b ("fortify: Add run-time WARN for cross-field memcpy()")
Tested-by: Martin-Éric Racine <martin-eric.racine@iki.fi>
Tested-by: Brandon Nielsen <nielsenb@jetfuse.net>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/ZuIhQRi/791vlUhE@decadent.org.uk