net: macb: use the current queue number for stats

There's a potential mismatch between the memory reserved for statistics
and the amount of memory written.

gem_get_sset_count() correctly computes the number of stats based on the
active queues, whereas gem_get_ethtool_stats() indiscriminately copies
data using the maximum number of queues, and in the case the number of
active queues is less than MACB_MAX_QUEUES, this results in a OOB write
as observed in the KASAN splat.

==================================================================
BUG: KASAN: vmalloc-out-of-bounds in gem_get_ethtool_stats+0x54/0x78
  [macb]
Write of size 760 at addr ffff80008080b000 by task ethtool/1027

CPU: [...]
Tainted: [E]=UNSIGNED_MODULE
Hardware name: raspberrypi rpi/rpi, BIOS 2025.10 10/01/2025
Call trace:
 show_stack+0x20/0x38 (C)
 dump_stack_lvl+0x80/0xf8
 print_report+0x384/0x5e0
 kasan_report+0xa0/0xf0
 kasan_check_range+0xe8/0x190
 __asan_memcpy+0x54/0x98
 gem_get_ethtool_stats+0x54/0x78 [macb
   926c13f3af83b0c6fe64badb21ec87d5e93fcf65]
 dev_ethtool+0x1220/0x38c0
 dev_ioctl+0x4ac/0xca8
 sock_do_ioctl+0x170/0x1d8
 sock_ioctl+0x484/0x5d8
 __arm64_sys_ioctl+0x12c/0x1b8
 invoke_syscall+0xd4/0x258
 el0_svc_common.constprop.0+0xb4/0x240
 do_el0_svc+0x48/0x68
 el0_svc+0x40/0xf8
 el0t_64_sync_handler+0xa0/0xe8
 el0t_64_sync+0x1b0/0x1b8

The buggy address belongs to a 1-page vmalloc region starting at
  0xffff80008080b000 allocated at dev_ethtool+0x11f0/0x38c0
The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000
  index:0xffff00000a333000 pfn:0xa333
flags: 0x7fffc000000000(node=0|zone=0|lastcpupid=0x1ffff)
raw: 007fffc000000000 0000000000000000 dead000000000122 0000000000000000
raw: ffff00000a333000 0000000000000000 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff80008080b080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff80008080b100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff80008080b180: 00 00 00 00 00 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
                                  ^
 ffff80008080b200: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffff80008080b280: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
==================================================================

Fix it by making sure the copied size only considers the active number of
queues.

Fixes: 512286bbd4b7 ("net: macb: Added some queue statistics")
Signed-off-by: Paolo Valerio <pvalerio@redhat.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260323191634.2185840-1-pvalerio@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>