osd/ReplicatedPG: allow CMPXATTR ops to trigger side effects too

osd/ReplicatedPG: allow CMPXATTR ops to trigger side effects too

This is for:
1. during the do_osd_ops() process, it may return 1 when processing
a CEPH_OSD_OP_CMPXATTR/CEPH_OSD_OP_SRC_CMPXATTR message to indicate
that the comparison result is positive. Therefore, the do_osd_ops()
method may also be able to return a value which is bigger than 0.
2. the ReplicatedPG::execute_ctx() method will try to catch any reads
operation and trigger do_osd_op_effects() on success, as below:

    // read or error?
    if (ctx->op_t->empty() || result < 0)
    {
        // finish side-effects
        if (result == 0)
            do_osd_op_effects(ctx, m->get_connection());

Thus we may don't call do_osd_op_effects() as expected if result is bigger
than 0 and op is not write.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>