When a driver needs to deal with a special buffer like a SG or a
vmalloc buffer, it has to set up the PCM page ops explicitly for the
corresponding helper function.  This is rather error-prone and many
people forgot or incorrectly used it.

For simplifying the call patterns and avoiding such a potential bug,
this patch enhances the PCM default mmap handler to check the
(pre-)allocated buffer type and handles the page gracefully depending
on the buffer type.  If the PCM page ops is given, the ops is still
used in a higher priority.  The new code path is only for the default
(NULL page ops) case.

Link: https://lore.kernel.org/r/20191105080138.1260-4-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>