include/cpp-btree: fix array bounds warning in child() accessors

Replace array indexing with pointer arithmetic in child() and
mutable_child() methods to avoid compiler warning when accessing
child nodes beyond the static array bounds.

The original code was functionally correct but triggered
-Warray-bounds when accessing mutable_child(32) during btree
operations. Using pointer arithmetic achieves the same result
without the bounds check warning.

This is a follow-up to commit 8458a19ab which fixed similar
warnings in other btree_node methods.

No functional changes.

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

Merge pull request #66489 from joscollin/wip-drop-double-check

mds: drop checking CEPH_MDS_OP_SETLAYOUT two times

Reviewed-by: Patrick Donnelly <pdonnell@ibm.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>

Merge pull request #66158 from xxhdx1985126/wip-seastore-minor-perf-issue

crimson/os/seastore/epm: avoid unnecessary container copies

Reviewed-by: Matan Breizman <mbreizma@redhat.com>
Reviewed-by: Samuel Just <sjust@redhat.com>

mds: drop checking CEPH_MDS_OP_SETLAYOUT two times

Signed-off-by: Jos Collin <jcollin@redhat.com>

Merge pull request #65773 from trociny/wip-69492

rbd-mirror: add cluster fsid to remote meta cache key

Reviewed-by: Ilya Dryomov <idryomov@gmail.com>