maple_tree: mte_set_full() and mte_clear_full() clang-analyzer clean up

mte_set_full() and mte_clear_full() were incorrectly setting a pointer to
a value without returning a result.  Fix this by returning the modified
pointer to be use as necessary.  Also add a third function to return if
the bit is set or not.

Link: https://lore.kernel.org/lkml/20221026120029.12555-1-lukas.bulwahn@gmail.com/
Link: https://lkml.kernel.org/r/20221028144520.2776767-1-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>