selftests/bpf: fix test_spin_lock_fail.c's global vars usage

selftests/bpf: fix test_spin_lock_fail.c's global vars usage

Global variables of special types (like `struct bpf_spin_lock`) make
underlying ARRAY maps non-mmapable. To make this work with libbpf's
mmaping logic, application is expected to declare such special variables
as static, so libbpf doesn't even attempt to mmap() such ARRAYs.

test_spin_lock_fail.c didn't follow this rule, but given it relied on
this test to trigger failures, this went unnoticed, as we never got to
the step of mmap()'ing these ARRAY maps.

It is fragile and relies on specific sequence of libbpf steps, which are
an internal implementation details.

Fix the test by marking lockA and lockB as static.

Fixes: c48748aea4f8 ("selftests/bpf: Add failure test cases for spin lock pairing")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241023043908.3834423-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>