LU-18518 ldiskfs: fix htree_lock array-index-out-of-bounds

LU-18518 ldiskfs: fix htree_lock array-index-out-of-bounds

This patch replaces 0-length trailing arrays with
flexible arrays in htree_lock.h to resolve the UBSAN
array-index-out-of-bounds runtime warnings.

Test-Parameters: mdtcount=4 mdscount=2 \
  clientdistro=ubuntu2404 testlist=sanity

Change-Id: I30b624a09f3f918ddaac105fb06db9fd4c693799
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57327
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Shuichi Ihara <sihara@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>