mei: vsc: Fix fortify-panic caused by invalid counted_by() use

mei: vsc: Fix fortify-panic caused by invalid counted_by() use

commit 00f1cc14da0f06d2897b8c528df7c7dcf1b8da50 upstream.

gcc 15 honors the __counted_by(len) attribute on vsc_tp_packet.buf[]
and the vsc-tp.c code is using this in a wrong way. len does not contain
the available size in the buffer, it contains the actual packet length
*without* the crc. So as soon as vsc_tp_xfer() tries to add the crc to
buf[] the fortify-panic handler gets triggered:

[   80.842193] memcpy: detected buffer overflow: 4 byte write of buffer size 0
[   80.842243] WARNING: CPU: 4 PID: 272 at lib/string_helpers.c:1032 __fortify_report+0x45/0x50
...
[   80.843175]  __fortify_panic+0x9/0xb
[   80.843186]  vsc_tp_xfer.cold+0x67/0x67 [mei_vsc_hw]
[   80.843210]  ? seqcount_lockdep_reader_access.constprop.0+0x82/0x90
[   80.843229]  ? lockdep_hardirqs_on+0x7c/0x110
[   80.843250]  mei_vsc_hw_start+0x98/0x120 [mei_vsc]
[   80.843270]  mei_reset+0x11d/0x420 [mei]

The easiest fix would be to just drop the counted-by but with the exception
of the ack buffer in vsc_tp_xfer_helper() which only contains enough room
for the packet-header, all other uses of vsc_tp_packet always use a buffer
of VSC_TP_MAX_XFER_SIZE bytes for the packet.

Instead of just dropping the counted-by, split the vsc_tp_packet struct
definition into a header and a full-packet definition and use a fixed
size buf[] in the packet definition, this way fortify-source buffer
overrun checking still works when enabled.

Fixes: 566f5ca97680 ("mei: Add transport driver for IVSC device")
Cc: stable@kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alexander Usyskin <alexander.usyskin@intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://lore.kernel.org/r/20250318141203.94342-2-hdegoede@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>