USB: g_file_storage: fix use-after-free bug when closing files

This patch (as1231) fixes a use-after-free bug in g_file_storage.  A
device's name may not be available after the device is unregistered,
even if the device structure itself is still allocated.  Since
close_backing_file() prints a LUN's name for debugging, it shouldn't
be called after the LUN has been unregistered.

That whole area needed to be cleaned up; the backing files were
getting closed in a couple of different places.  The patch fixes
things so that they get closed in just one place, as part of the
unbind procedure, immediately before the LUN is unregistered.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>