maple_tree: Prefilled sheaf conversion and testing

Use prefilled sheaves instead of bulk allocations. This should speed up
the allocations and the return path of unused allocations.

Remove the push and pop of nodes from the maple state as this is now
handled by the slab layer with sheaves.

Testing has been removed as necessary since the features of the tree
have been reduced.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>