lockd: introduce nlmsvc_serv

lockd has two globals - nlmsvc_task and nlmsvc_rqst - but mostly it
wants the 'struct svc_serv', and when it doesn't want it exactly it can
get to what it wants from the serv.

This patch is a first step to removing nlmsvc_task and nlmsvc_rqst.  It
introduces nlmsvc_serv to store the 'struct svc_serv*'.  This is set as
soon as the serv is created, and cleared only when it is destroyed.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>