usb: gadget: f_mass_storage: create _fsg_common_free_buffers

When configfs is in place, gadgets will have to be able to free
fsg buffers. Add a helper function.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>