ceph: fix buffer pointer advance in ceph_sync_write

We should advance the user data pointer by _len_ instead of _written_.
_len_ is the data length written in each iteration while _written_ is the
accumulated data length we have writtent out.

Signed-off-by: Henry C Chang <henry.cy.chang@gmail.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Tested-by: Sage Weil <sage@inktank.com>