x86: Unspaghettize do_general_protection()

There is some unnatural label based layout in this function.
Convert the unnecessary goto to readable conditional blocks.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>