pcm: pcm_ioplug - fix the avail_update mmap capture copy issue

pcm: pcm_ioplug - fix the avail_update mmap capture copy issue

It seems that the commit "pcm: ioplug: Transfer all available data"
introduced new regressions (wrong memory access). The second issue
is that the avail_update in ioplug does not move appl_ptr nor hw_ptr,
so it's possible that the transfers may be repetitive.

This patch moves the transfer calls to mmap_begin callback where it
should be. The pointer wraps are handled by design now.

Fixes: 1714332719 ("pcm: ioplug: Transfer all available data")
BugLink: https://github.com/alsa-project/alsa-lib/pull/103
Tested-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>