ReplicatedPG::do_osd_ops: only return ENOTSUP on OMAP write ops

This way, redirected omap reads will work correctly by returning
empty maps and buffers.

Fixes: #7870
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1536 from ceph/wip-7870

ReplicatedPG::do_osd_ops: only return ENOTSUP on OMAP write ops

Reviewed-by: Sage Weil <sage@inktank.com>