USB: gadget: file_storage: put_device() in error recovery

This commit fixes some issues with File-backed Storage Gadget
error recovery when registering LUN's devices.

First of all, when device_register() fails the device still
needs to be put.  However, because lun_release() decreases
fsg->ref reference counter the counter must be incremented
beforehand.

Second of all, after any of the device_create_file()s fails,
device_unregister() is called which in turn (indirectly) calls
lun_release() which decrements fsg->ref.  So, again, the
reference counter must be incremented beforehand.

Lastly, if the first or the second device_create_file()
succeeds, the files are never removed.  To fix it,
device_remove_file() needs to be called.  This is done by
simply marking LUN as registered prior to creating files so
that fsg_unbind() can handle removing files.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Reported-by: Rahul Ruikar <rahul.ruikar@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>