ALSA: hda: Allow setting preallocation again for x86

The commit c31427d0 ("ALSA: hda: No preallocation on x86
platforms") changed CONFIG_SND_HDA_PREALLOC_SIZE setup and its default
to zero for x86, as the preallocation should work almost all cases.
However, this expectation was too naive; some applications try to
allocate as the max buffer size as possible, and it leads to the
memory exhaustion.  More badly, the commit changed the kconfig no
longer adjustable for x86, so you can't fix it statically (although it
can be still adjusted via procfs).

So, practically seen, it's more recommended to set a reasonable limit
for x86, too.  This patch follows to that experience, and changes the
default to 2048 and allow the kconfig adjustable again.

Fixes: c31427d0 ("ALSA: hda: No preallocation on x86 platforms")
Cc: <stable@vger.kernel.org>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=207223
Link: https://lore.kernel.org/r/20200413201919.24241-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/Kconfig

@@ -21,16 +21,17 @@ config SND_HDA_EXT_CORE
        select SND_HDA_CORE
 
 config SND_HDA_PREALLOC_SIZE
-	int "Pre-allocated buffer size for HD-audio driver" if !SND_DMA_SGBUF
+	int "Pre-allocated buffer size for HD-audio driver"
 	range 0 32768
-	default 0 if SND_DMA_SGBUF
+	default 2048 if SND_DMA_SGBUF
 	default 64 if !SND_DMA_SGBUF
 	help
 	  Specifies the default pre-allocated buffer-size in kB for the
 	  HD-audio driver.  A larger buffer (e.g. 2048) is preferred
 	  for systems using PulseAudio.  The default 64 is chosen just
 	  for compatibility reasons.
-	  On x86 systems, the default is zero as we need no preallocation.
+	  On x86 systems, the default is 2048 as a reasonable value for
+	  most of modern systems.
 
 	  Note that the pre-allocation size can be changed dynamically
 	  via a proc file (/proc/asound/card*/pcm*/sub*/prealloc), too.