bpf: Move insn if/else into do_check_insn()

bpf: Move insn if/else into do_check_insn()

This is required to catch the errors later and fall back to a nospec if
on a speculative path.

Eliminate the regs variable as it is only used once and insn_idx is not
modified in-between the definition and usage.

Do not pass insn but compute it in the function itself. As Eduard points
out [1], insn is assumed to correspond to env->insn_idx in many places
(e.g, __check_reg_arg()).

Move code into do_check_insn(), replace
* "continue" with "return 0" after modifying insn_idx
* "goto process_bpf_exit" with "return PROCESS_BPF_EXIT"
* "goto process_bpf_exit_full" with "return process_bpf_exit_full()"
* "do_print_state = " with "*do_print_state = "

[1] https://lore.kernel.org/all/293dbe3950a782b8eb3b87b71d7a967e120191fd.camel@gmail.com/

Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Henriette Herzog <henriette.herzog@rub.de>
Cc: Maximilian Ott <ott@cs.fau.de>
Cc: Milan Stephan <milan.stephan@fau.de>
Link: https://lore.kernel.org/r/20250603205800.334980-2-luis.gerhorst@fau.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>