crimson/common: add alternative overload for FixedKVNodeLayout<>::iter_t

crimson/common: add alternative overload for FixedKVNodeLayout<>::iter_t

otherwise we'd have following FTBFS with C++20 and clang13:

In file included from /var/ssd/ceph/src/crimson/os/seastore/lba_manager.cc:7:
In file included from /var/ssd/ceph/src/crimson/os/seastore/lba_manager/btree/btree_lba_manager.h:24:
/var/ssd/ceph/src/crimson/os/seastore/lba_manager/btree/lba_btree.h:474:26: error: use of overloaded operator '==' is ambiguous (with operand types 'crimson::common::FixedKVNodeLayout<254, crimson::os::seastore::l$
    auto end = next_iter == parent->end()
               ~~~~~~~~~ ^  ~~~~~~~~~~~~~
/var/ssd/ceph/src/crimson/common/fixed_kv_node_layout.h:127:10: note: candidate function
    bool operator==(const iter_t &rhs) const {
         ^
/var/ssd/ceph/src/crimson/common/fixed_kv_node_layout.h:127:10: note: candidate function (with reversed parameter order)

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