ALSA: pcm: Workaround for a wrong offset in SYNC_PTR compat ioctl

Michael Forney reported an incorrect padding type that was defined in
the commit 80fe7430 ("ALSA: add new 32-bit layout for
snd_pcm_mmap_status/control") for PCM control mmap data.
His analysis is correct, and this caused the misplacements of PCM
control data on 32bit arch and 32bit compat mode.

The bug is that the __pad2 definition in __snd_pcm_mmap_control64
struct was wrongly with __pad_before_uframe, which should have been
__pad_after_uframe instead.  This struct is used in SYNC_PTR ioctl and
control mmap.  Basically this bug leads to two problems:

- The offset of avail_min field becomes wrong, it's placed right after
  appl_ptr without padding on little-endian

- When appl_ptr and avail_min are read as 64bit values in kernel side,
  the values become either zero or corrupted (mixed up)

One good news is that, because both user-space and kernel
misunderstand the wrong offset, at least, 32bit application running on
32bit kernel wor...