LU-17426 mdt: relax same MDT file rename lock

LU-17426 mdt: relax same MDT file rename lock

Allow cross-directory rename of regular files (strictly, any
non-directory) on the same MDT without holding the BigFilesystemLock
(BFL), as file renames cannot change the directory hierarchy.

This should improve the performance for these rename operations, and
reduce contention between local MDT file renames in different parts of
the directory tree.

Add "mdt.*.enable_parallel_rename_crossdir" parameter to disable
cross-directory file renames if there is an issue with this change.

Lustre-change: https://review.whamcloud.com/53726
Lustre-commit: TBD (from d466465f9add30faec256ce7f725e0f36d4e8a66)

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I511b392e46c46140cac6aa3ede02bfe793729f7f
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53744
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>