selftests/bpf: Test an arena kfunc argument passed from a subprogram

Call a kfunc with an __arena argument from a global subprogram that does
nothing else with the arena, the address arrives as a number and no cast
is involved. The subprogram needs the arena base set up by its own
prologue for the JIT to rebase the argument, which the BPF CI review
found was not recorded for it.

Pass a NULL constant to a nullable __arena kfunc argument as well, from
a program that touches the arena nowhere else. The verifier takes its
early exit for a NULL argument, but the JIT rebases the call like any
other, behind the NULL test the function model asks for, so the arena
base has to be set up for that call alone.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-17-memxor@gmail.com

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