h_TossStuff_r: make sure host does not go away

h_TossStuff_r: make sure host does not go away

When h_TossStuff_r h_NBLock_r's a host, it is not only possible for
someone else to grab a hold on the host, but in theory it's also
possible for someone to hold a host, release it, and for the host to be
deleted again (assuming some callers hold HOSTDELETED hosts, which they
should not be doing).

To make this safety check a bit more robust, hold the host in
h_TossStuff_r before h_NBLock_r'ing, to ensure that it does not go away
while we're waiting for H_LOCK.

Change-Id: I018a611143d6dccba79cc627803cce74aa830c77
Reviewed-on: http://gerrit.openafs.org/1330
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>