net: gro_cells: Use nested-BH locking for gro_cell

The gro_cell data structure is per-CPU variable and relies on disabled
BH for its locking. Without per-CPU locking in local_bh_disable() on
PREEMPT_RT this data structure requires explicit locking.

Add a local_lock_t to the data structure and use
local_lock_nested_bh() for locking. This change adds only lockdep
coverage and does not alter the functional behaviour for !PREEMPT_RT.

Reported-by:  <syzbot+8715dd783e9b0bef43b1@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68c6c3b1.050a0220.2ff435.0382.GAE@google.com/


Fixes: 3253cb49 ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20251009094338.j1jyKfjR@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>