mkdir() and rmdir() during resync can cause races between bulk sync and
mod sync that can lead to failed resyncs or even data loss.
if a client runs mkdir() that creates an inode in inodes/$L1/$L2 after
bulk sync for inodes/$L1/$L2 has synced all inodes, but before (or
while) bulk sync is removing all inodes it has not transferred itself,
the new inode may be erroneously deleted. if a client runs rmdir()
instead, the deleted inode may cause a resync failure when bulk sync
tries to remove a file that is no longer present.
(cherry picked from commit a0c307c513a7697be03ddd0c203c6e612a936053)