USB: g_file_storage: add CD-ROM emulation

This patch (as1172) adds the ability to emulate a CD-ROM drive to
g_file_storage.  The emulation is limited, since it presents as a disc
containing a single data track and no audio tracks.  Still, it may
come in useful on occasion.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>