qcow2: Fix L1 write error handling in qcow2_update_snapshot_refcount

It ignored the error code, and at least the 'goto fail' is obvious
nonsense as it creates an endless loop (if the next attempt doesn't
magically succeed) and leaves the in-memory L1 table in big-endian
instead of converting it back.

In error cases, there's no point in writing an updated L1 table, so
skip this part for them.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>