rbd: set mapping name with the rest

With the exception of the snapshot name, all of the mapping-specific
fields in an rbd device structure are set in rbd_header_set_snap().

Pass the snapshot name to be assigned into rbd_header_set_snap()
to keep all of the mapping assignments together.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>