xen-netback: fix debugfs entry creation

The original code is bogus. The function gets called in a loop which
leaks entries created in previous rounds.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>