vfs: add cwd check to umount to prevent filesystem in use

vfs: add cwd check to umount to prevent filesystem in use

Added busy check in vfs_umount_nolock to reject unmount if any process
has its current working directory (cwd) within the mount being unmounted.

Implementation details:
- Checks current_process->cwd first (process executing umount)
- Iterates over ready_queue_head to check all other processes
- Uses path_is_mountpoint_prefix to determine if cwd is within mount
- Returns -EBUSY if any process has cwd in the mount
- Note: root directory check not implemented (would require vfs_getpath
  or storing root as path in process struct)

Also added test I20 (umount cwd) to fulltest.c:
- Mounts tmpfs to /tmp/mnt_cwd
- Changes cwd into the mount
- Verifies umount fails with -EBUSY
- Changes cwd back to /
- Verifies umount succeeds

Updated test harnesses (smoke_test.exp, test_battery.exp) to include
the new test pattern.

Test results:
- Smoke test: 124/124 PASS
- Zero regressions