staging: most: 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

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20191108164528.998-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>