ALSA: provide a more useful get_unmapped_area handler for pcm

Shared memory mappings on nommu machines require a get_unmapped_area
file operation that suggests an address for the mapping. The current
implementation returns 0 and thus forces the driver to implement an
mmap handler that fixes up the start and end address of the vma.

This patch returns the address of the dma buffer, so it should work
out of the box for all drivers that use the snd_pcm_runtime->dma_area
pointer.

Addresses for mapping the status and control pages are returned as
well, but to make those work the conditional compilation of
snd_pcm_mmap_{status,control} would need to be revised.

URL: http://thread.gmane.org/gmane.linux.alsa.devel/61230
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>