kfifo: fix kerneldoc for kfifo_avail and kfifo_in_locked

Function argument is not having "len" anywhere;
"len" is later used in kfifo_{in|out}.

So here, it would be appropriate to say,
it copies @n bytes and not @len

Signed-off-by: Viral Mehta <viral.mehta@lntinfotech.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>