ALSA: aloop - Use vmalloc buffer

snd-aloop driver is virtual and has no need for allocating contiguous
pages.  It'll be more system-friendly to use vmalloc buffers.

Tested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>