maple_tree: Replace mt_free_one() with kfree()

kfree() is a little shorter and works with kmem_cache_alloc'd pointers
too. Also lets us remove one more helper.

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>