lockd: Add nlm_destroy_host_locked()

Refactor the tail of nlm_gc_hosts() into nlm_destroy_host() so that
this logic can be used separately from garbage collection.

Rename it _locked() to document that it must be called with the hosts
cache mutex held.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>