iov_iter is a universal interface to copy the data chunk from/to
user-space and kernel in a unified manner.  This API can fit for ALSA
PCM copy ops, too; we had to split to copy_user and copy_kernel in the
past, and those can be unified to a single ops with iov_iter.

This patch adds a new PCM copy ops that passes iov_iter for copying
both kernel and user-space in the same way.  This patch touches only
the ALSA PCM core part, and the actual users will be replaced in the
following patches.

The expansion of iov_iter is done in the PCM core right before calling
each copy callback.  It's a bit suboptimal, but I took this now as
it's the most straightforward replacement.  The more conversion to
iov_iter in the caller side is a TODO for future.

As of now, the old copy_user and copy_kernel ops are still kept.
Once after all users are converted, we'll drop the old copy_user and
copy_kernel ops, too.

Link: https://lore.kernel.org/r/20230815190136.8987-3-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>