maple_tree: mas_anode_descend() clang-analyzer cleanup

clang-analyzer reported some Dead Stores in mas_anode_descend().  Upon
inspection, there were a few clean ups that would make the code cleaner:

The count variable was set from the mt_slots array and then updated but
never used again.  Just use the array reference directly.

Also stop updating the type since it isn't used after the update.

Stop setting the gaps pointer to NULL at the start since it is always
set before the loop begins.

Link: https://lkml.kernel.org/r/20221026151413.4032730-1-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>