A rename within one directory locks the target name's PDO bucket only
when the directory is local to the serving MDT (mdt_lock_two_dirs(),
the !mdt_object_remote() check). A same-directory rename is served on
the MDT of the file at the target name when one exists, so when that
file lives on another MDT than the directory, the rename holds the
directory's CW lock and the source name's bucket only. The target
name can then change between the rename's lookup of it and its child
locks, and nothing looks it up again: mdd_rename() deletes the target
entry by name.

Since LU-15526 (7270e16fcb, 2.16.0) a PW lock on a remote parent is a
per-name PDO lock; before it, it was a whole-directory lock that
covered the target name. From then on an unlink and re-create of the
target name can change it in the window. Since LU-17427 (8dbf049479,
2.17.0) the BFL is taken after the child locks, so another rename
onto the same name can too.

Effect, two clients renaming different files onto one name:

• If the file at the target name has no other link, the second rename
  fails with ENOENT and changes nothing. POSIX requires it to succeed,
  so an application replacing a file atomically (write temp, rename
  over) sees an unexpected error.
• If that file has another hard link, the second rename deletes the
  entry that now names the first rename's file and inserts its own.
  The first rename's file is left with nlink 1 and no name (an orphan,
  visible to LFSCK only), and the file the second rename looked up is
  destroyed while its other hard link still names it. Both renames
  return 0.

Reproduced on master with a fail point holding the second rename after
its target lookup (directory on MDT0, files on MDT1, A/dst hard linked
as A/keep; mv A/a A/dst and mv A/b A/dst from two mounts): A/dst names
the second file, A/keep no longer resolves, and the first file is an
orphan. The unlink/re-create route follows from the source and is not
reproduced.

Fix: lock the target name's bucket for a remote directory too, in hash
order as LU-15285 does for a local one. Patch, with regression test
sanityn 81j: https://review.whamcloud.com/c/fs/lustre-release/+/69434

The parallel cross-MDT file rename change (LU-20831,
https://review.whamcloud.com/c/fs/lustre-release/+/69203) widens the
window further by removing the BFL from these renames, and is stacked
on top of the fix.