[PATCH] USB: file-storage gadget: Add reference count for children

This patch (as601) adds a proper reference count to the file-storage
gadget's main data structure, to keep track of references held by child
devices (LUNs in this case).  Before this, the driver would wait for
each child to be released before unbinding.

While there's nothing really wrong with that (you can't create a hang by
doing "rmmod g_file_storage </sys/.../lun0/ro" since the open file will
prevent rmmod from running), the code might as well follow the standard
procedures.  Besides, this shrinks the size of the structure by a few
words...  :-)

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