nfsd: Reduce the callback timeout to 800msec

Recent discussion on the nfsv4@ietf.org mailing list confirmed
that an NFSv4 server should reply to an RPC in less than 1second.
If an NFSv4 RPC requires a delegation be recalled,
the server will attempt a CB_RECALL callback.
If the client is not responsive, the RPC reply will be delayed
until the callback times out.
Without this patch, the timeout is set to 4 seconds (set in
ticks, but used as seconds), resulting in the RPC reply taking over 4sec.
This patch redefines the constant as being in milliseconds and it
implements that for a value of 800msec, to ensure the RPC
reply is sent in less than 1second.

This patch only affects mounts from clients when delegations
are enabled on the server and the client is unresponsive to callbacks.

MFC after: 2 weeks