ALSA: memalloc: Align buffer allocations in page size

Currently the standard memory allocator (snd_dma_malloc_pages*())
passes the byte size to allocate as is.  Most of the backends
allocates real pages, hence the actual allocations are aligned in page
size.  However, the genalloc doesn't seem assuring the size alignment,
hence it may result in the access outside the buffer when the whole
memory pages are exposed via mmap.

For avoiding such inconsistencies, this patch makes the allocation
size always to be aligned in page size.

Note that, after this change, snd_dma_buffer.bytes field contains the
aligned size, not the originally requested size.  This value is also
used for releasing the pages in return.

Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201218145625.2045-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>