crimson/os/seastore/lba_manager: clarify ref count operation return

crimson/os/seastore/lba_manager: clarify ref count operation return

Previously, we returned a refcount from inc_ref and dec_ref.  Now,
return the paddr as well for future code accounting for released
extents.

In addition, replumb btree_lba_manager to return an enoent error if
the mapping does not exist, and the resulting refcount, paddr
otherwise with a refcount of 0 indicating that the mapping has
been removed.

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