USB: g_mass_storage: most data moved to fsg_common

Most of the data from fsg_dev have been moved to fsg_common
structure.  The fsg_dev structure holds only endpoint dependent
data.  The fsg_common structure has a fsg pointer which points
to active fsg_dev structure -- endpoints are referenced via this
pointer.

This fixes the problem of several threads created when a single
instance of MSF is used in several USB configurations.
Signed-off-by: NMichal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>