maple_tree: shrink struct maple_tree

Pack the members of struct maple_tree to avoid holes on 64-bit.  The size
shrinks from 24 to 16 bytes which will save eight bytes in every structure
which embeds it.

[willy@infradead.org: changelog alterations]
Link: https://lkml.kernel.org/r/20230821225145.2169848-1-mjguzik@gmail.com
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>