USB: g_mass_storage: template f_mass_storage.c file created

Copied file_storage.c to f_mass_storage.c which will be used as
template for the Mass Storage composite Function.

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>