USB: g_mass_storage: parts of fsg_dev moved to fsg_common structure

In the final version, many fsg_dev structures will (be able to)
refer to a single fsg_common structure and so it is required
to move common data to another object which can be shared.

Situation where many fsg_dev structures refer single fsg_common
structure is when a single instance of MSF is used in several
USB configurations.
Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>