lockd: define host_for_each{_safe} macros

We've got a lot of loops like this, and I find them a little easier to
read with the macros.  More such loops are coming.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
[ cel: Forward-ported to 2.6.37 ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>