usb-gadget-ether: prevent oops caused by error interrupt race

Fix a longstanding race in the Ethernet gadget driver, which can cause an
oops on device disconnect.  The fix is just to make the TX path check
whether its freelist is empty.  That check is otherwise not necessary,
since the queue is always stopped when that list empties (and restarted
when request completion puts an entry back on that freelist).

The race window starts when the network code decides to transmit a packet,
and ends when hard_start_xmit() grabs the freelist lock.  When disconnect()
is called inside that window, it shuts down the TX queue and breaks the
otherwise-solid assumption that packets are never sent through a TX queue
that's stopped.

Signed-off-by: Benedikt Spranger <bene@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>