bpf: Support pointer param types via SCALAR_VALUE for trampolines

bpf: Support pointer param types via SCALAR_VALUE for trampolines

Add BPF verifier support for single- and multi-level pointer
parameters and return values in BPF trampolines by treating these
parameters as SCALAR_VALUE.

This extends the existing support for int and void pointers that are
already treated as SCALAR_VALUE.

This provides consistent logic for single and multi-level pointers:
if a type is treated as SCALAR for a single-level pointer, the same
applies to multi-level pointers. The exception is pointer-to-struct,
which is currently PTR_TO_BTF_ID for single-level but treated as
scalar for multi-level pointers since the verifier lacks context
to infer the size of target memory regions.

Safety is ensured by existing BTF verification, which rejects invalid
pointer types at the BTF verification stage.

Signed-off-by: Slava Imameev <slava.imameev@crowdstrike.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260314082127.7939-2-slava.imameev@crowdstrike.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>