x86/traps: Introduce fill_trap_info(), simplify DO_ERROR_INFO()

Extract the fill-siginfo code from DO_ERROR_INFO() into the new helper,
fill_trap_info().

It can calculate si_code and si_addr looking at trapnr, so we can remove
these arguments from DO_ERROR_INFO() and simplify the source code. The
generated code is the same, __builtin_constant_p(trapnr) == T.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>