maple_tree: simplify mas_push_node()

maple_tree: simplify mas_push_node()

When count is not 0, we know head is valid.  So we can put the assignment
in if (count) instead of checking the head pointer again.

Also count represents current total, we can assign the new total by
increasing the count by one.

Link: https://lkml.kernel.org/r/20241015120746.15850-4-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>