ceph: kill ceph_osdc_writepages() "flags" parameter

There is only one caller of ceph_osdc_writepages(), and it always
passes 0 as its "flags" argument.  Get rid of that argument and
replace its use in ceph_osdc_writepages() with 0.

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