[JFFS2] Fix potential memory leak of dead xattrs on unmount.

An xattr_datum which ends up orphaned should be freed by the GC
thread. But if we umount before the GC thread is finished, or if we
mount read-only and the GC thread never runs, they might never be
freed. Clean them up during unmount, if there are any left.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>