libdm: dm_tree_node_size_changed recognizes reduction

libdm: dm_tree_node_size_changed recognizes reduction

Add more functionality to size_changed function.
While 'existing' API only detected  0 for
unchanged,  and !0 for changed,
new improved API will also detected if the
size has only went bigger - or there was
size reduction.

Function work for the whole dm-tree - so
no change is size is always 0.
only size extension  1.
and if some size reduction is there - returns -1.

This result can be used for better evaluation
whether we need to flush before suspend.