net: stmmac: Fix accessing freed irq affinity_hint

net: stmmac: Fix accessing freed irq affinity_hint

The cpumask should not be a local variable, since its pointer is saved
to irq_desc and may be accessed from procfs.
To fix it, use the persistent mask cpumask_of(cpu#).

Cc: stable@vger.kernel.org
Fixes: 8deec94c6040 ("net: stmmac: set IRQ affinity hint for multi MSI vectors")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250318032424.112067-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>