usb: gadget: f_mass_storage: create lun creation helpers for use in fsg_common_init

fsg_common_init is a lengthy function. Factor portions of it out.

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>