ALSA: control: Simplify snd_ctl_elem_list() implementation

ALSA: control: Simplify snd_ctl_elem_list() implementation

This patch simplifies the code of snd_ctl_elem_list() in the following
ways:

- Avoid a vmalloc() temporary buffer but do copy in each iteration;
  the vmalloc buffer was introduced at the time we took the spinlock
  for the ctl element management.

- Use the standard list_for_each_entry() macro

- Merge two loops into one;
  it used to be a loop for skipping until offset becomes zero and
  another loop to copy the data.  They can be folded into a single
  loop easily.

Reviewed-by: Takashi Sakamoto <[email protected]>
Tested-by: Takashi Sakamoto <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>