cpp-btree: fix array bounds warning in btree_node::swap()

cpp-btree: fix array bounds warning in btree_node::swap()

Replace direct array indexing with pointer arithmetic to avoid
compiler warning when forming ranges for std::swap_ranges().

The original code was functionally correct but triggered
-Werror=array-bounds when accessing mutable_child(count + 1)
to create the end iterator. Using pointer arithmetic achieves
the same result without the bounds check warning.

No functional changes.

Fixes: https://tracker.ceph.com/issues/72477
Signed-off-by: Kefu Chai <tchaikov@gmail.com>