The recent refactoring of mmap handling caused Oops on some devices
that don't use the standard memory allocations.  This patch addresses
it by allowing snd_dma_buffer_mmap() helper to receive the NULL
pointer dmab argument (and return an error appropriately).

Fixes: a202bd1a ("ALSA: core: Move mmap handler into memalloc ops")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211107163911.13534-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>