pcm: dshare: Fix overflow when slave_hw_ptr rolls over boundary

pcm: dshare: Fix overflow when slave_hw_ptr rolls over boundary

In snd_pcm_dshare_sync_area() when 'slave_hw_ptr' rolls over
'slave_boundary', the wrong variable is checked ('dshare->slave_hw_ptr' vs
the local 'slave_hw_ptr'). In some cases, this results in 'slave_hw_ptr'
not rolling over correctly. 'slave_size' and 'size' are then much too
large, and the for loop blocks for several minutes copying samples.

This was likely only triggered on 32-bit systems, since the PCM boundary
is computed based on LONG_MAX and is much larger on 64-bit systems.

This same change was made to pcm_dmix in commit
6c7f60f7a982fdba828e4530a9d7aa0aa2b704ae ("Fix boundary overlap”) from
June 2005.

Signed-off-by: Brendan Shanks <brendan.shanks@teradek.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>