rpc.idmapd: Ignore open failures in dirscancb()

From: David Jeffery <djeffery@redhat.com>

The daemon "rpc.idmapd" scans the /var/lib/nfs/rpc_pipefs/nfs/ directory
periodically looking for NFS client mounts to communicate to. The daemon
tried to open communication with a client mount but it disappeared in
between looking for directory entries and opening them. NFS mount was
umounted just before rpc.idmapd tried to communicate with it. This
behavior is usually seen when autofs is configured on the system.

Signed-off-by: Steve Dickson <steved@redhat.com>