bpf: Allow a global function to return arena pointers by value

A global function may already return an arena pointer on its own, and
check_global_ret_scalar_reg() accepts one in either half of the R0:R2
pair. Let the members of a by-value struct it returns be arena pointers
as well, rather than scalars only.

Only a subprogram gets this: the main program returns to the kernel,
which has no arena to cast the address back into, so
btf_validate_return_type() keeps rejecting an arena pointer there,
whether bare or inside a struct. The rejection message therefore has to
name what the function at hand may return, or it would offer the main
program an arena pointer it cannot have.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061550.1696869-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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