bpf: Record each half of a paired return value in verifier diagnostics

A subprogram returning more than 8 bytes comes back in the R0:R2 register
pair. There are three places, check_func_call(), prepare_func_exit() and
check_kfunc_call(), where only R0 is tracked by the diagnostic
modification scope and R2 is missed.

Fix it by opening a diagnostic modification scope for each return register.
This way, both return registers are recorded.

Fixes: 0630ad00d96d ("bpf: Add verifier support for 16-byte returns in R0: R2")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061519.1692270-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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