lockd: Add nlm_alloc_host()

Refactor nlm_host allocation and initialization into a separate
function.  This will be the common piece of server and client nlm_host
lookup logic after the nlm_host cache is split.

Small change: use kmalloc() instead of kzalloc(), as we're overwriting
almost all fields in the new nlm_host struct with non-zero values
immediately after it is allocated.  An added benefit is we now have an
explicit reference to each field name where it is initialized (for all
you cscope fans out there).

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