maple_tree: clean up mas_wr_append()

Avoid setting the variables until necessary, and actually use the
variables where applicable.  Introducing a variable for the slots array
avoids spanning multiple lines.

Add the missing argument to the documentation.

Use the node type when setting the metadata instead of blindly assuming
the type.

Finally, add a trace point to the function for successful store.

Link: https://lkml.kernel.org/r/20230819004356.1454718-3-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>