bpf: Shrink size of struct bpf_map/bpf_array.

Back in 2018 the commit be95a845 ("bpf: avoid false sharing of map refcount
with max_entries") added ____cacheline_aligned to "struct bpf_map" to make sure
that fields like refcnt don't share a cache line with max_entries that is used
to bounds check map access. That was done to make spectre style attacks harder.
The main mitigation is done via code similar to array_index_nospec(), of course.
This was an additional precaution.

It increased the size of "struct bpf_map" a little, but it's affect on all
other maps (like array) is significant, since "struct bpf_map" is typically
the first member in other map types.

Undo this ____cacheline_aligned tag. Instead move freeze_mutex field around, so
that refcnt and max_entries are still in different cache lines.

The main effect is seen in sizeof(struct bpf_array) that reduces from 320
to 248 bytes.

BEFORE:

struct bpf_map {
	const struct bpf_map_ops  * ops;                 /*     0     8 */
	...
	char                       name[16];             /*    96    16 */

	/* XXX 16 bytes hole, try to pack */

	/* --- cacheline 2 boundary (128 bytes) --- */
	atomic64_t refcnt __attribute__((__aligned__(64))); /*   128     8 */
	...
	/* size: 256, cachelines: 4, members: 30 */
	/* sum members: 232, holes: 1, sum holes: 16 */
	/* padding: 8 */
	/* paddings: 1, sum paddings: 2 */
} __attribute__((__aligned__(64)));

struct bpf_array {
	struct bpf_map             map;                  /*     0   256 */
	...
	/* size: 320, cachelines: 5, members: 5 */
	/* padding: 48 */
	/* paddings: 1, sum paddings: 8 */
} __attribute__((__aligned__(64)));

AFTER:

struct bpf_map {
	/* size: 232, cachelines: 4, members: 30 */
	/* paddings: 1, sum paddings: 2 */
	/* last cacheline: 40 bytes */
};
struct bpf_array {
	/* size: 248, cachelines: 4, members: 5 */
	/* last cacheline: 56 bytes */
};

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240220235001.57411-1-alexei.starovoitov@gmail.com