Reapply "bnxt_en: bring back rtnl_lock() in the bnxt_open() path"

Reapply "bnxt_en: bring back rtnl_lock() in the bnxt_open() path"

This reverts commit 850d9248d2eac662f869c766a598c877690c74e5.
This reapplies commit 325eb217e41f ("bnxt_en: bring back rtnl_lock()
in the bnxt_open() path").

Breno reports a lockdep warning in bnxt. During FW reset the driver
may end up calling netif_set_real_num_tx_queues() (if queue count
changes), so calls to bnxt_open() still require rtnl_lock.

  net/sched/sch_generic.c:1416 suspicious rcu_dereference_protected() usage!

   dev_qdisc_change_real_num_tx+0x54/0xe0
   netif_set_real_num_tx_queues+0x4ed/0xa80
   __bnxt_open_nic+0x9cb/0x3490
   bnxt_open+0x1cb/0x370
   bnxt_fw_reset_task+0x80d/0x1e80
   process_scheduled_works+0x9c1/0x13b0

The reverted commit was just an optimization / experiment
so let's go back to taking the lock.

Reported-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/ah726OtFX-Qw3U-R@gmail.com
Fixes: 850d9248d2ea ("Revert "bnxt_en: bring back rtnl_lock() in the bnxt_open() path"")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260603195845.2574426-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>