ALSA: aloop: Convert to the common vmalloc memalloc

The recent change (*) in the ALSA memalloc core allows us to drop the
special vmalloc-specific allocation and page handling.  This patch
coverts to the common code.
(*) 1fe7f397: ALSA: memalloc: Add vmalloc buffer allocation
                  support
    7e8edae3: ALSA: pcm: Handle special page
                  mapping in the default mmap handler

Link: https://lore.kernel.org/r/20191105151856.10785-10-tiwai@suse.de


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