dm thin metadata: Remove unused local variable when create thin and snap

The local variable disk details is not used during the creating of thin & snap
devices. Remove them from dm-thin-metadata, and add pointer validity check for
pointer value in btree_lookup_raw. Skip memory copy when the caller doesn't need
the value.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>