usb: f_mass_storage: test whether thread is running before starting another

When binding the function to usb_configuration, check whether the thread
is running before starting another one.  Without that, when function
instance is added to multiple configurations, fsg_bing starts multiple
threads with all but the latest one being forgotten by the driver.  This
leads to obvious thread leaks, possible lockups when trying to halt the
machine and possible more issues.

This fixes issues with legacy/multi¹ gadget as well as configfs gadgets
when mass_storage function is added to multiple configurations.

This change also simplifies API since the legacy gadgets no longer need
to worry about starting the thread by themselves (which was where bug
in legacy/multi was in the first place).

N.B., this patch doesn’t address adding single mass_storage function
instance to a single configuration twice.  Thankfully, there’s no
legitimate reason for such setup plus, if I’m not mistaken, configfs
gadget doesn’t even allow it to be expressed.

¹ I have no example failure though.  Conclusion that legacy/multi has
  a bug is based purely on me reading the code.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Tested-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>