usb: gadget: f_midi: added spinlock on transmit function

Since f_midi_transmit is called by both ALSA and USB sub-systems, it can
potentially cause a race condition between both calls because f_midi_transmit
is not reentrant nor thread-safe. This is due to an implementation detail that
the transmit function looks for the next available usb request from the fifo
and only enqueues it if there is data to send, otherwise just re-uses it. So,
if both ALSA and USB frameworks calls this function at the same time,
kfifo_seek() will return the same usb_request, which will cause a race
condition.

To solve this problem a syncronization mechanism is necessary. In this case it
is used a spinlock since f_midi_transmit is also called by usb_request->complete
callback in interrupt context.

Cc: <stable@vger.kernel.org> # v4.5+
Fixes: e1e3d7ec

 ("usb: gadget: f_midi: pre-allocate IN requests")
Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>