ceph: fix direct-io on non-page-aligned buffers

The user buffer may be 512-byte aligned, not page-aligned.  We were
assuming the buffer was page-aligned and only accounting for
non-page-aligned io offsets.

Signed-off-by: Henry C Chang <henry_c_chang@tcloudcomputing.com>
Signed-off-by: Sage Weil <sage@newdream.net>