libceph: page offset must be less than page size

Currently ceph_msg_data_pages_advance() allows the page offset value
to be PAGE_SIZE, apparently assuming ceph_msg_data_pages_next() will
treat it as 0.  But that doesn't happen, and the result led to a
helpful assertion failure.

Change ceph_msg_data_pages_advance() to truncate the offset to 0
before returning if it reaches PAGE_SIZE.

Make a few other minor adjustments in this area (comments and a
better assertion) while modifying it.

This resolves a second issue described in:
    http://tracker.ceph.com/issues/4598



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>