bpf: Fix sizeof(arg_type) based global subprog stack access

Allow global subprogs "read" poisoned stack slots.
The static stack liveness determined that subprog doesn't read certain
stack slots, but sizeof(arg_type) based global subprog validation
isn't accurate enough to know which slots will actually be read by
the callee, so it needs to check full sizeof(arg_type) at the caller.

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