libceph: set page alignment in start_request()

The page alignment field for a request is currently set in
ceph_osdc_build_request().  It's not needed at that point
nor do either of its callers need that value assigned at
any point before they call ceph_osdc_start_request().

So move that assignment into ceph_osdc_start_request().

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