ALSA: seq: obsolete change of address limit

Former commits change existent functions so that they don't handle data in
kernel space. Copying from/to userspace is done outside of the functions,
thus no need to change address limit of running task.

This commit obsoletes get_fs()/set_fs() and applies corresponding changes.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>