avcodec/v4l2_context: use EAGAIN to signal when input buffers are unavailable

ENOMEM indicates an allocation failure, and there are no allocations
happening here. The buffers are pre-allocated and there are simply
none available at this time.
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/v4l2_context.c

@@ -572,7 +572,7 @@ int ff_v4l2_context_enqueue_packet(V4L2Context* ctx, const AVPacket* pkt)
 
     avbuf = v4l2_getfree_v4l2buf(ctx);
     if (!avbuf)
-        return AVERROR(ENOMEM);
+        return AVERROR(EAGAIN);
 
     ret = ff_v4l2_buffer_avpkt_to_buf(pkt, avbuf);
     if (ret)

libavcodec/v4l2_m2m_dec.c

@@ -142,7 +142,7 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
 
     ret = ff_v4l2_context_enqueue_packet(output, &avpkt);
     if (ret < 0) {
-        if (ret != AVERROR(ENOMEM))
+        if (ret != AVERROR(EAGAIN))
            return ret;
         /* no input buffers available, continue dequeing */
     }