lockd: get host reference in nlmsvc_create_block() instead of callers

It may not be obvious (till you look at the definition of
nlm_alloc_call()) that a function like nlmsvc_create_block() should
consume a reference on success or failure, so I find it clearer if it
takes the reference it needs itself.

And both callers already do this immediately before the call anyway.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>