Comment 23 for bug 1073089

Assuming you meant "linux-image-extra-<version>-generic" - this does resolve the problem. (there is no package linux-kernel-extras).

Additionally, although the gadgetfs module is present, mounting it fails:

al@al-desktop:~$ sudo mount -t gadgetfs none /dev/gadget
mount: unknown filesystem type 'gadgetfs'

I believe this is happening because the module dummy_hcd is missing. This module is required for gadget_fs to function on x86 platforms which do not have any real gadget capable hardware.