cxgb4: Properly lock TX queue for the selftest.

The selftest for the driver sends a dummy packet and checks if the
packet will be received properly as it should be. The regular TX path
and the selftest can use the same network queue so locking is required
and was missing in the selftest path. This was addressed in the commit
cited below.
Unfortunately locking the TX queue requires BH to be disabled which is
not the case in selftest path which is invoked in process context.
Lockdep should be complaining about this.

Use __netif_tx_lock_bh() for TX queue locking.

Fixes: c650e048

 ("cxgb4: Fix race between loopback and normal Tx path")
Reported-by: "John B. Wyatt IV" <jwyatt@redhat.com>
Closes: https://lore.kernel.org/all/Zic0ot5aGgR-V4Ks@thinkpad2021/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20240429091147.YWAaal4v@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>