usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers() Previously fsg_num_buffers_validate() was removed as it was not necessary due to Kconfig setting the limits for n from 2 to 256 with default as 2. However, setting the page content in such a way that kstrtou8() reflects n value as either 0 or 1 bypasses these restrictions leading to a null pointer dereference if n is 0. Fix this by adding a check for n < 2 and returning -EINVAL if n is either 0 or 1 consistent with Kconfig logic.