maple_tree: Support freeing empty nodes on gap relocation.

maple_tree: Support freeing empty nodes on gap relocation.

When a gap is created during a low memory situation, coalesce may not be
able to group the gap together.  This can be handled by moving the right
node to the left & using a SKIP entry.  Note that this situation should
not arise unless in a low-memory situation, so a SKIP entry is useful as
it indicates the tree may need to be rebuilt after memory pressure
subsides.

When an empty node is found,

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>