USB: g_mass_storage: lun_name_format and thread_name added

A two fsg_config fields were added:
* lun_name_format which lets one specify format of a name
  used when registering LUN devices.  It is useful if there
  would be ever need for two MSFs to be used in a single
  composite gadget (as opposed to single MSF in two
  configuration); and
* thread_name which lets one specify the name of a kernel
  thread used by MSF.  This is not required since two or more
  threads can have the same name but nevertheless it's here
  for consistency.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>