selftests/bpf: Fix a user_ringbuf failure with arm64 64KB page size

selftests/bpf: Fix a user_ringbuf failure with arm64 64KB page size

[ Upstream commit bbc7bd658ddc662083639b9e9a280b90225ecd9a ]

The ringbuf max_entries must be PAGE_ALIGNED. See kernel function
ringbuf_map_alloc(). So for arm64 64KB page size, adjust max_entries
properly.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250607013626.1553001-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>