NFS: Clear key construction data if the idmap upcall fails

NFS: Clear key construction data if the idmap upcall fails

idmap_pipe_downcall already clears this field if the upcall succeeds,
but if it fails (rpc.idmapd isn't running) the field will still be set
on the next call triggering a BUG_ON().  This patch tries to handle all
possible ways that the upcall could fail and clear the idmap key data
for each one.

Signed-off-by: Bryan Schumaker <[email protected]>
Tested-by: William Dauchy <[email protected]>
Cc: [email protected] [>= 3.4]
Signed-off-by: Trond Myklebust <[email protected]>