libceph: define and use ceph_crc32c_page()

Factor out a common block of code that updates a CRC calculation
over a range of data in a page.

This and the preceding patches are related to:
    http://tracker.ceph.com/issues/4403



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>