selftests/bpf: fix flaky build_id test

selftests/bpf: fix flaky build_id test

The build_id selftest occasionally fails because MADV_PAGEOUT
does not guarantee the immediate eviction of the page. The test
assumes eviction happens and proceeds without verifying
that the page was actually reclaimed, leading to false test
failures.

Fix the test by retrying the page-out sequence until eviction
is successful, instead of relying on a single MADV_PAGEOUT attempt.

Signed-off-by: Gregory Bell <grbell@redhat.com>
Link: https://lore.kernel.org/r/038bd27c69dd3a16958894fcb19e4fb6fbfe317e.1771338492.git.grbell@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>