LU-11085 ldlm: move interval_insert call from ldlm_lock to ldlm_extent

LU-11085 ldlm: move interval_insert call from ldlm_lock to ldlm_extent

Moving this call results in all interval-tree handling code
being in the one file. This will simplify conversion to
use Linux interval trees.

The addition of 'struct cb' is a little ugly, but will be gone
is a subsequent patch.

Change-Id: I7b392cc57b69969f4bb3c4b51fa406ed643a37b3
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/34021
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>