sound: Force-disable hw.snd.compat_linux_mmap by default

Eventually this hack will be removed, so start by disabling it by
default.

Sponsored by:	The FreeBSD Foundation
MFC after:	12 months
Reviewed by:	emaste
Pull Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/29

share/man/man4/pcm.4

@@ -267,7 +267,7 @@ call, which
 .Fx
 does not.
 .Pp
-The following values are supported (default is 0):
+The following values are supported (default is -1):
 .Bl -tag -width 2n
 .It -1
 Force-disable PROT_EXEC

sys/dev/sound/pcm/dsp.c

@@ -55,7 +55,7 @@ struct dsp_cdevpriv {
 };
 
 #ifdef SV_ABI_LINUX
-static int dsp_mmap_allow_prot_exec = 0;
+static int dsp_mmap_allow_prot_exec = -1;
 SYSCTL_INT(_hw_snd, OID_AUTO, compat_linux_mmap, CTLFLAG_RWTUN,
     &dsp_mmap_allow_prot_exec, 0,
     "linux mmap compatibility (-1=force-disable 0=auto)");