libceph: don't set pages or bio in ceph_osdc_alloc_request()
Only one of the two callers of ceph_osdc_alloc_request() provides
page or bio data for its payload. And essentially all that function
was doing with those arguments was assigning them to fields in the
osd request structure.
Simplify ceph_osdc_alloc_request() by having the caller take care of
making those assignments
Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>