maple_tree: Add mtree_alloc_cyclic()

I need a cyclic allocator for the simple_offset implementation in
fs/libfs.c.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/170820144179.6328.12838600511394432325.stgit@91.116.238.104.host.secureserver.net


Signed-off-by: Christian Brauner <brauner@kernel.org>