x86: traps_xx: restructure do_general_protection()

 - if (cond) block -> if (!cond) goto end_of_block
 - local caching of current

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>