selftests/bpf: Cover the tail call caller stack depth limit

A tail call from a subprog only unwinds that subprog's frame, so the
verifier refuses tail calls once the frames of the callers add up to
256 bytes or more. Nothing exercised that rule. Add a pair of tests
with a caller using 240 and 256 bytes of stack respectively, the
latter expecting the "tail_calls are not allowed when call stack of
previous frames is 256 bytes" rejection.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260924165740.2146806-10-memxor@gmail.com

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