sched_ext: Cleanup [__]scx_exit/error*()

sched_ext: Cleanup [__]scx_exit/error*()

__scx_exit() is the base exit implementation and there are three wrappers on
top of it - scx_exit(), __scx_error() and scx_error(). This is more
confusing than helpful especially given that there are only a couple users
of scx_exit() and __scx_error(). To simplify the situation:

- Make __scx_exit() take va_list and rename it to scx_vexit(). This is to
  ease implementing more complex extensions on top.

- Make scx_exit() a varargs wrapper around __scx_exit(). scx_exit() now
  takes both @kind and @exit_code.

- Convert existing scx_exit() and __scx_error() users to use the new
  scx_exit().

- scx_error() remains unchanged.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>