crimson/os/seastore/lba_manager/btree: move duplicate_for_write

crimson/os/seastore/lba_manager/btree: move duplicate_for_write

Previously, it appears that I meant for the caller to insert to
invoke duplicate_for_write prior to calling insert().  I think
I chose that convention to avoid chained callbacks within a method
not on *this.  However, there are two scenarios where this is
necessary: leaf and split/merge.  The split/merge criteria are a
little involved and were only buggily considered.

This patch moves responsibilty for duplicate_for_write on *this
into insert/update.  Callers should assume that the callee will
handle it, and should not assume that they can call back into the
same node.

Signed-off-by: Samuel Just <sjust@redhat.com>