bpf: Move fixup/post-processing logic from verifier.c into fixups.c

bpf: Move fixup/post-processing logic from verifier.c into fixups.c

verifier.c is huge. Split fixup/post-processing logic that runs after
the verifier accepted the program into fixups.c.

Mechanical move. No functional changes.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260412152936.54262-2-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>