lockd: shift grabbing a reference to nlm_host into nlm_alloc_call()

It's used both for client and server hosts; we can't do nlmclnt_release_host()
on failure exits, since the host might need nlmsvc_release_host(), with BUG_ON()
for calling the wrong one.  Makes life simpler for callers, actually...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>