crimson/os/seastore: use iter_cend()

crimson/os/seastore: use iter_cend()

to avoid the pain of fixing the FTBFS with C++20 like

/var/ssd/ceph/src/crimson/os/seastore/omap_manager/btree/omap_btree_node_impl.cc:141:19: error: use of overloaded operator '!=' is ambiguous (with operand types 'crimson::os::seastore::omap_manager::StringKVInnerN$
  assert(child_pt != iter_end());
         ~~~~~~~~ ^  ~~~~~~~~~~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/var/ssd/ceph/src/crimson/os/seastore/omap_manager/btree/string_kv_node_layout.h:380:10: note: candidate function
    bool operator==(const iter_t &rhs) const {
         ^
/var/ssd/ceph/src/crimson/os/seastore/omap_manager/btree/string_kv_node_layout.h:380:10: note: candidate function (with reversed parameter order)

also, to be explicit is advisable.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>