net/9p: fix error path of p9_virtio_probe

Currently when virtio_find_single_vq fails, we go through del_vqs which
throws a warning (Trying to free already-free IRQ).  Skip del_vqs if vq
allocation failed.

Link: http://lkml.kernel.org/r/20180524101021.49880-1-jean-philippe.brucker@arm.com


Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>