Track subdirectory count per-inode in memory via InodeEntry.subdirCount.
Increment on mkdir, decrement on rmdir, adjust on cross-directory
rename. applyDirNlink uses this count instead of listing metacache
entries, so nlink is correct immediately after mkdir without needing
a prior readdir.
Remove tests/rename/24.t from known_failures.txt (all 13 subtests
now pass).