LU-12542 handle: discard OBD_FREE_RCU

OBD_FREE_RCU and the hop_free call-back together form an overly
complex mechanism equivalent to kfree_rcu() or call_rcu(...).
Discard them and use the simpler approach.

This removes the only use for the field h_size, so discard
that too.

Change-Id: I3b4135565dab6a9aa5034f42ae3f9b66851cae31
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35797
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>