osd: Perform shard look up correctly in partial EC writes

osd: Perform shard look up correctly in partial EC writes

Plugins are permitted to provide a mapping to change the order in which OSDs
are used. In practice only LRC does this and it is not currently enabled
with optimisations, so this is a theoretical bug.

The bug here was that the "first" shard was assumed to be shard_id_t(0).  However,
this is not true for LRC.

Fixes: https://tracker.ceph.com/issues/74016
Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>