LU-11085 ldlm: save space in struct ldlm_lock

LU-11085 ldlm: save space in struct ldlm_lock

Moving the 'interval' handle into ldlm_lock has made the structure
bigger.  Compensate for this by shared space for fields only needs for
specific lock types.

i.e.  some fields are only needed for EXTENT locks, some for FLOCK
locks, some for PLAIN and IBITS which use "skiplists".

One x86_64 the reduces the size of ldlm_lock to what is was before the
previous patch.  A future patch will reduce it even more.

As extent and flock both used the interval tree node, they now have
different instances.  So the names in flock are changed.  Both of
these will disappear in future patches.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Iec92a41c174e4884852ebf8fbb2cd50d4e165035
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53931
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>