LU-17276 flock: fix for lock split

LU-17276 flock: fix for lock split

Fix the logic in lock split case. Also add test
case to verify it works well.

Fixes: ea012dfb2a (LU-17276 ldlm: use interval tree for searching in flock)
Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I9599592eba217ce79956c45c0bbb58a7f5e805d0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57105
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>