rbd: plug rbd_dev->header.object_prefix memory leak
rbd: plug rbd_dev->header.object_prefix memory leak

Need to free object_prefix when rbd_dev_v2_snap_context() fails, but
only if this is the first time we are reading in the header.

Signed-off-by: Ilya Dryomov <152d4c5bebf13a5b809d42b4d789cf9d887e6698@gmail.com>
Reviewed-by: Alex Elder <f429030cf5c0faf36fac3d102073b6e63a647baa@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs