usb: gadget: midi2: fix missing unlock in f_midi2_block_opts_create()

In the error path in f_midi2_block_opts_create(), mutex_unlock() is missed,
fix it by move the unlock after 'out' label.

Fixes: 29ee7a4d ("usb: gadget: midi2: Add configfs support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20230728085120.3192474-1-yangyingliang@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>