bpf: Allow 'may_goto 0' instruction in verifier

Commit 011832b9 ("bpf: Introduce may_goto instruction") added support
for may_goto insn. The 'may_goto 0' insn is disallowed since the insn is
equivalent to a nop as both branch will go to the next insn.

But it is possible that compiler transformation may generate 'may_goto 0'
insn. Emil Tsalapatis from Meta reported such a case which caused
verification failure. For example, for the following code,
   int i, tmp[3];
   for (i = 0; i < 3 && can_loop; i++)
     tmp[i] = 0;
   ...

clang 20 may generate code like
   may_goto 2;
   may_goto 1;
   may_goto 0;
   r1 = 0; /* tmp[0] = 0; */
   r2 = 0; /* tmp[1] = 0; */
   r3 = 0; /* tmp[2] = 0; */

Let us permit 'may_goto 0' insn to avoid verification failure for codes
like the above.

Reported-by: Emil Tsalapatis <etsal@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250118192024.2124059-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>