usb: gadget: f_midi2: fix use-after-free in string attribute show path

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_midi2: fix use-after-free in string attribute show path f_midi2_opts_str_show() takes the string lock internally, but its callers dereference the opts->info.<field> pointer before calling it, outside the lock. This races with f_midi2_opts_str_store(), which frees the old string under opts->lock when the attribute is written concurrently, the show path can read a pointer that gets freed before the lock inside str_show() is even taken. Change f_midi2_opts_str_show() to take a pointer to the string field, matching the existing pattern in f_midi2_opts_str_store(), and dereference it only after the lock is held. Update all three callers (iface_name, block name, and the EP string option macro) accordingly.