ALSA: memalloc: Add fallback SG-buffer allocations for x86

The recent change for memory allocator replaced the SG-buffer handling
helper for x86 with the standard non-contiguous page handler.  This
works for most cases, but there is a corner case I obviously
overlooked, namely, the fallback of non-contiguous handler without
IOMMU.  When the system runs without IOMMU, the core handler tries to
use the continuous pages with a single SGL entry.  It works nicely for
most cases, but when the system memory gets fragmented, the large
allocation may fail frequently.

Ideally the non-contig handler could deal with the proper SG pages,
it's cumbersome to extend for now.  As a workaround, here we add new
types for (minimalistic) SG allocations, instead, so that the
allocator falls back to those types automatically when the allocation
with the standard API failed.

BTW, one better (but pretty minor) improvement from the previous
SG-buffer code is that this provides the proper mmap support without
the PCM's page fault handling.

Fixes: 2c95b92e

 ("ALSA: memalloc: Unify x86 SG-buffer handling (take#3)")
BugLink: https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/2272
BugLink: https://bugzilla.suse.com/show_bug.cgi?id=1198248
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220413054808.7547-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>