maple_tree: Add single node allocation support to maple state

The fast path through a write will require replacing a single node in
the tree.  Using a sheaf (32 nodes) is too heavy for the fast path, so
special case the node store operation by just allocating one node in the
maple state.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>