SNDCTL_DSP_GETIPTR: set pointer to sndbuf_getfreeptr()

Rationale:
- unlike current behavior this seems to be compliant with OSS
  specification:
  http://manuals.opensound.com/developer/SNDCTL_DSP_GETIPTR.html
- this seems to meet expectations of some OSS programs compiled for or
  ported from Linux, e.g. ALSA OSS plugin
- this doesn't seem to break any programs as far as current testing
  shows

Tested by:	nox, hselasky
MFC after:	4 days

sys/dev/sound/pcm/dsp.c

@@ -1655,7 +1655,7 @@ dsp_ioctl(struct cdev *i_dev, u_long cmd, caddr_t arg, int mode,
 				/* XXX abusive DMA update: chn_rdupdate(rdch); */
 	        		a->bytes = sndbuf_gettotal(bs);
 	        		a->blocks = sndbuf_getblocks(bs) - rdch->blocks;
-	        		a->ptr = sndbuf_getreadyptr(bs);
+	        		a->ptr = sndbuf_getfreeptr(bs);
 				rdch->blocks = sndbuf_getblocks(bs);
 				CHN_UNLOCK(rdch);
 	    		} else