lockd: host: Remove unnecessary statements＇host = NULL;＇

In 'nlm_alloc_host', the host has already been assigned a value of NULL
when defined, so 'host=NULL;' Can be deleted.

Signed-off-by: Li kunyu <kunyu@nfschina.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>