Maintain global unused node/entry lists, and per-mount used/unused entry lists

The global lists will be used for freeing unused but cached entries and nodes,
and the per-mount entry lists will be used by kern_fs_unmount() to determine
if a mount is in use and to free up all entries on a mount.

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Fix bugs descending through mountpoints in dentry KDB command

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>