selftests/bpf: Rename libarena malloc/free methods

The s390 architecture uses the token "free" for an enum, conflicting
with the malloc/free definitions. Rename the calls to arena_malloc and
arena_free instead to prevent collisions.

Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Emil Tsalapatis <etsal@meta.com>
Fixes: 86426a28c52d ("selftests/bpf: Add buddy allocator for libarena")
Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260428134252.2783519-1-etsal@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>