maple_tree: introduce mas_tree_parent() definition

Add a definition to shorten long code lines and clarify what the code is
doing.  Use the new definition to get the maple tree parent pointer from
the maple state where possible.

Link: https://lkml.kernel.org/r/20230804165951.2661157-5-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>