vfs: implement cwd busy check via mount refcount

vfs: implement cwd busy check via mount refcount

Implemented mount refcount-based busy check for umount to prevent
filesystem unmount when processes have cwd inside the mount.

Implementation uses mount refcount approach (same as Linux/BSD):
- Added vfs_mount_ref_by_path() and vfs_mount_unref_by_path()
- Functions use longest prefix match to find most specific mount
- chdir() updates refcounts (unref old cwd, ref new cwd)
- Process creation increments refcount for initial cwd
- Process exit decrements refcount for cwd
- vfs_umount_nolock() rejects if refcount > 0

This approach is O(1) for the check, avoids storing root as path
string (which caused struct alignment issues), and matches how
commercial OSes handle mount busy detection.

Test I20 (umount cwd) validates the implementation:
- Mounts tmpfs to /tmp/mnt_cwd
- Changes cwd into mount
- Verifies umount fails with -EBUSY
- Changes cwd back to /
- Verifies umount succeeds

Test results: 124/124 PASS