ALSA: pcm: Create proc files only for non-empty preallocations

It makes little sense to create prealloc proc files for streams that
have the zero max size, which is a typical case for vmalloc buffers.
Skip the proc file creations to save resources in such a case.

Link: https://lore.kernel.org/r/20191105191007.18150-3-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>