ALSA: pcm: Fix mmap without buffer preallocation

The recent rewrite of the memory allocation helpers also changed the
page extraction to a common helper, snd_sgbuf_get_page().  But this
assumes implicitly that the buffer was allocated via the standard
helper (usually via preallocation), and didn't consider the case of
the manual buffer handling.

This patch fixes it and also covers the manual buffer management.

Fixes: 37af81c5 ("ALSA: core: Abstract memory alloc helpers")
Link: https://lore.kernel.org/r/20210720092732.12412-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>