osd: Make EC Sync reads return a zero RC for success.

osd: Make EC Sync reads return a zero RC for success.

This was a misunderstanding of the API. The librados client expects that
a read returns zero on success, not the number of bytes read.

Correspondingly, simplify the client-based SplitOps rc handling code to
always return 0. It tolerates >0 RCs, but will always return zero.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>